"Los doctores dicen que es seguro."
Translation:The doctors say that it is safe.
Both "it" and "that" are indefinite pronouns in English, guob. That is, both stand for some other thing, which could be an idea or a physical object. The difference is that "it" can only be used as a pronoun. "That," however, can be used as a pronoun, as a type of adjective, and also as a type of conjunction (subordinate) introducing a dependent clause. So, in the examples "that it is safe" and "that it can only be used as a pronoun," the word "that" is the subordinating conjunction that turns the independent clauses "it is safe" and "it can only be used as a pronoun" into dependent clauses. For the record, both of your examples mean much the same thing.
One simple reason why "we" as students should assume that is because this is a learning program and in this particular lesson module the sentence truly isn't part of a regular conversation. That being expressed miza713. What you expressed is certainly valid in the context/scenario that you presented it in. :)
DL accepted "the doctors say it is certain," which means a very different thing. While both may be technically correct possible meanings, I would think Spanish speakers would find a different sentence structure or word to say my version. (I sort of think mine should have been marked wrong.)
For the purposes of learning the way people use the language, it seems DL sometimes doesn't accept weird phrasings—phrasings that make sense in English, but simply would be said differently in Spanish to mean the same thing. I think I get why they do this, and I would have expected this to be one of those cases, but apparently it's not. shrug You're not wrong, and neither is my translation technically, but that's not my point.
The word stable in translated as estable based on my findings. I would suggest you not put too much emphasis on the options shown because they aren't specific to any particular sentence. I have on hand a dictionary when I'm in doubt. This is one of the few times I can't find any reference in 3 dictionaries to seguro = stable. Strange and possibly a mistake.