Rival
Opponent
Foe
Ally
Answer is Wrong!
Answer is Right!
The answer is D. Ally.
A rival is someone who competes with you for the same thing. An opponent is someone who is against you in a contest or argument. A foe is an enemy. An ally is a person or group that is associated with another in a friendly or cooperative relationship.
In the given options, all are words that refer to someone who is against you or competes with you, except for ally, which refers to someone who is on your side.