Correct Answer: Accomplice

Solution : The fourth option is the correct choice.

Accomplice refers to a person who helps another commit a crime.

The meanings of the other options are:

• Agent : A person who acts on behalf of another person or group.
• Amateur : A person who engages in a pursuit, especially a sport, on an unpaid rather than professional basis.
• Alibi : A claim or piece of evidence that one was elsewhere when an act, typically a criminal one, is alleged to have taken place.

Hence, from the given meanings, we find that accomplice is the correct one-word substitute.