He believes __________ honesty.

This question tests your knowledge of correct prepositional usage, specifically with the verb 'believe.' The idiomatic expression 'believe in' is used to convey faith, trust, or confidence in the existence, truth, or value of something or someone. When you 'believe in honesty,' it means you have faith in the principle of honesty, you trust its value, or you believe that it is a good and true quality. Other prepositions like 'on,' 'at,' or 'for' do not form a grammatically correct or meaningful phrase with 'believe' in this context. For instance, 'believe on' is incorrect, 'believe at' is not used in this sense, and 'believe for' would imply a reason rather than an object of faith. Therefore, 'in' is the only preposition that correctly completes the sentence, forming the standard and widely accepted idiom 'believe in honesty,' which signifies a conviction in the virtue of honesty.