Who's responsible?

Good guess. FAR 91.409(a) requires an annual, performed and returned to service by an A&P holding Inspection Authorization. Day-by-day inspections are not a mechanic's responsibility. FAR 91.7(b) puts the determination of whether the aircraft is in condition for safe flight on one person: the pilot in command. Option C is the answer.

This is the trap, and it's a good one. FAR 91.403(a) does say the owner or operator is primarily responsible for maintaining the aircraft in an airworthy condition, including AD compliance. But FAR 91.7(b) says the pilot in command is responsible for determining whether that aircraft is in condition for safe flight. Option C is the answer.

Correct. FAR 91.7(b): the pilot in command is responsible for determining whether the aircraft is in condition for safe flight (and there can only be one acting PIC for any flight).

This was close. But FAR 91.7(b) names the pilot in command, singular, and 91.3(a) makes the PIC directly responsible for, and the final authority as to, the operation of the aircraft. Another pilot can help with preflight, read the checklist, and catch what the PIC missed, but there can only be one Acting PIC. Option C is the answer.