English usage – negotiation collocation and noun choice: complete the sentence so it reads naturally and idiomatically about trust in talks. Sentence frame: 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. Choose the single most appropriate noun that collocates with "in … sincerity" without changing the intended meaning.

Given data

• Sentence frame: 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.
• Options: faith, belief, substance, certainty.

Concept/Approach
In diplomatic English, the idiomatic collocation expressing trust is have faith in someone's sincerity. This matches both grammar (noun + in) and meaning (confidence, trust).

Option analysis
faith — forms the set phrase faith in (someone's) sincerity; precise and natural. ✅belief — grammatical with in but less idiomatic for interpersonal sincerity in negotiations. ❌substance — means real content; substance in the sincerity is awkward and semantically off. ❌certainty — typically with about/that; also too absolute for negotiations. ❌

Common pitfalls

• Confusing faith vs. belief: both can take in, but only faith collocates smoothly with sincerity to mean trust.

Final Answer
faith