"Cynic' and _ are synonyms:

Explanation

The word 'cynic' refers to a person who doubts or questions the sincerity or motives of others, often believing that humans are motivated by self-interest. The correct synonym is 'skeptic', which also denotes a person who questions or doubts accepted opinions. The other options do not align with this meaning: 'secret' is unrelated to questioning or doubting, 'solitary' pertains to being alone, and 'truthful' relates to honesty rather than skepticism.