Correct Answer: turncoat

Solution : The third option is the correct answer.

Explanation : A turncoat is someone who changes sides, especially in politics or allegiance, often by betraying a former cause or party. This term encapsulates the idea of a person who abandons one side or group and joins the opposing one.

The meanings of the other options are as follows:

• Invader : This term refers to someone who enters a place forcefully or as an aggressor.
• Novice : This term refers to a person who is new or inexperienced in a particular activity or field.
• Amateur : This term refers to someone who engages in an activity for pleasure.

Therefore, the correct one-word substitute is turncoat .