Magniloquent means speaking in a grandiose, pompous manner. Bombastic is an adjective for the language itself, and Sesquipedalian refers specifically to long words, not the speaker's manner.

Solipsist believes only their own mind exists. Nihilist denies all meaning, Existentialist focuses on existence and freedom, and Relativist denies absolute truths.

Misogamy (miso=hatred, gamy=marriage) means hatred of marriage. Gamophobia means fear of marriage (phobia=fear), while Misogyny means hatred of women.

Chauvinist shows excessive, aggressive devotion to one's country (from Nicolas Chauvin). Patriot loves country normally; Nationalist emphasizes national identity; Loyalist shows allegiance to ruler/government.

'Caught off guard' means surprised or unprepared for something unexpected.

'Beat around the bush' is correctly used. It means to avoid discussing the main point.

'Spill the beans' means to reveal a secret or tell someone something that was meant to be kept confidential.

'Piece of cake' means something is very easy to do. Option A correctly applies this idiom.

'Cost an arm and a leg' means something is very expensive. Option A uses it correctly.

'See eye to eye' is correctly used. It means to agree with someone.