Fortunately, here it is the gramatical "то" which can never be это. So, no real choice for you :)
то + что always go together like that. Это is not used grammatically with что and который.
A brief review of grammar: when a subordinate clause works as a whole as a member of the main clause, you might have a problem sometimes. Namely, when it works as an object that is not in the Nominative.
Let's imagine you want to say "I do not believe what he told us about".
You look up "believe" in a dictionary and see that the meaning you'd like to use is expressed as верить + Dative. You also know that "what he told us about" is "о чём он нам (рас)сказал". However, "о чём" is positively NOT Dative, and the Dative"чему" will not work in "tell about something". In English "what" is perfectly fine for both clauses—apparently, in Russian it is not.
To solve this, we insert a dummy "то" as follows:
- Я не верю тому, о чём он нам рассказал.
Now, «то» does its job in the main clause and «что» works in the subordinate clause—each taking an appropriate form for the grammar you used.
(of course, you could also use верить в or you could rephrase to "I do not believe his words")
There is another situation where you need it. Very rarely do native speakers say things like "About what he told us, was very interesting". Starting a sentence like that feels very awkward. Instead, we use the dummy то and, again, get "That which he told us about was very interesting":
- То, о чём он нам рассказал, было очень интересно.
There are two uses for the verb: you can tell someone something or you can tell someone about something.
The person receiving the information will indeed be in the Dative. The information being told is in the Accusative. If you tell about something, it is о + Prepositional. It is really the property of "о".
Ah so the problem comes from combining the верить + Dative with the second part then right? Without the тому it would be "я не верю, о чём он нам рассказал" which is incorrect because the thing after верить needs to be in dative but the phrase we're using requires чём to be prepositional. If that's the case then that does indeed make perfect sense.
The adjective правый (of which the права here is a feminine singular short-form adjective) means both "correct" and "right-hand". Many words are based on the root: справа (located statically on the right), направо (directional movement to the right), право (a right as in law, права is the plural form...so you have to know the context in order to get the meaning correct!).
Isn't it interesting how correctness is connected to sides? In English, dextrous is from the Latin for right (side) - dexter - and sinister from the Latin for left (side) - sinistra. From French we have adroit (competent, from droit = right) and gauche (awkward, gauche = left)
In phrases of this general type (I wish, Suppose, etc) "that" is optional.
But "the thing is" is typically followed by a comma and no "that". A comma would have been good here.
Note: this is standard U.S. and British English. Used for example by Jane Austen in Northanger Abbey.
I think of this phrase as:
The deal/concern (дело)
in that [matter] (в том)
(is) that (, что)
she is right (она права).
In literal English, it's confusing because "том, что" can be translated as "that, that" - but each "that" has a different grammatical function: the first "that" is an indefinite pronoun, and the second "that" is a conjunction.
The first "that" = Том is the indefinite pronoun. As the object of the preposition "в", it is in prepositional case and references an unstated and hence unknown set of circumstances about which she is correct. (Nominative case is "то".)
The second "that" = что is a conjunction which simply unites the first clause with the second.
I wrote: Дела в том, что она права in listening. I refuse to accept that this is completely wrong given that unstressed о and a literally sound the same. Unfortunately you can't report listening exercises.
P.S. I know it's grammatically incorrect I just think it's stupid to count it as a major error in a listening exercise.
It is not completely wrong—yet since you heard the wrong form, the only reasonable deduction is, you could not understand what was being said. Sure, "Eye never red that book" should not be acceptable in the course teaching English?
It would be OK if both versions were meaningfull Russian sentences.
Just because they sound the same it doesn't mean that it's ok to write the grammatically incorrect version. You are not just mindlessly writing whatever you hear, but you're actually supposed to understand what was said. I mean, for people who are learning English, "hair", "heir" and "hare" might also sound the same, but that would be a silly reason for insisting on using them interchangeably.