"He demonstrates that in his book."
Translation:Il démontre ça dans son livre.
"Ce" is either a subject or an adjective. It is not a direct object that goes after a verb.
@William. Il le/la demontre should be accepted... Depending on contexte, if arguing, depending where emphasis is: is it on what was demonstrated, or on wether or not it was demonstrated... Without context, as a native speaker myself I could go with either/or demontre ça/ ceci/cela, le/la demontre, ...
“Il montre” is a correct translation according to my French dictionary. It should be accepted.
"WHAT" is correct? He demonstrates "THAT" this is correct: Il demontre "QUE" cela est correct. (far from speaker)
"WHICH" is better? He demonstrates "THAT" that is better: Il demontre "QUE" ceci est mieux. (close to speaker)
"WHAT" did he demonstrate? He demonstrate "THAT" it's correct : il démontre que c'est correct (distance does not matter)
"WHAT" does he demonstrate in his book? He demonstrates this or that in his book: il démontre ceci/cela dans son livre (no relative "THAT")
In English, "THAT" can be a relative pronoun, similar to "WHAT", "WHICH", "WHO". "QUE" in French It combines two clauses in one sentence: He said THAT this is not war = he said: "this is not war" He saw the man THAT he wanted to marry = he saw a man, he wanted to marry him. He saw WHOM he wanted to marry = he saw someone, he wanted to marry him/her. Sometimes the "THAT" can be ignored, but it is implied: he saw the girl he wanted to marry. (In English, but not in French!)
But in the sentence, there is no relative clause: no French "que" is needed. The "that" in that sentence is not a relative pronoun, it just replaced a noun: "This is my coat", "That is correct", "It's better", "She is correct", "It is a coat", "This is not a pipe", "This will be his wife".
The pronoun "ça" as a direct object does not go before the verb.