What is the purpose of an audit?

Doing a practice accounting exam and want to double check my work. The question is about the definition of an audit, however the book doesn't mention an audit at all so can't really check my work.

Question: The purpose of an audit is to..

Answers:

a) determine whether or not a company is a good investment

b) render an opinion on the fairness of the statements

c) determine whether or not a company complies with income tax regulations

d) determine whether or not a company is a good credit risk

I know almost nothing about audits as we have not covered them yet, and there is no mention of them in the book so I am at a loss. My guess was C but the word "opinion" got me concerned.

Thanks for any and all help you can provide!