Answer: C. Easily defeated
Explanation: Invincible itself means one who cannot die from unnatural causes (but can still die from old age, etc.)
Keep in mind, invincible does NOT mean immortal. Immortal means you can never die. Invincible just means you cannot die or be defeated from unnatural causes like people fighting you.
An antonym is an opposite of a word. The opposite of being undefeated is a person who is easily defeated. Thus the antonym for invincible is "C," easily defeated.