If negotiations are to prove fruitful, there must not only be sincerity on each side, but there must also be ...... in the sincerity of the other side.

To understand the sentence, it is important to recognize that it talks about the necessity of belief or confidence in the sincerity of the other side for negotiations to be successful. Among the options provided, the word that best fits this context is faith.

So, the sentence should read: "If negotiations are to prove fruitful, there must not only be sincerity on each side, but there must also be faith in the sincerity of the other side."