Choose the word which is different from the rest.

The answer is A. Rival.

A rival is someone who competes with someone else for the same thing, especially in a game or sport. A spouse is a husband or wife. A partner is someone who shares an activity or business with another person. A colleague is a person who works in the same organization as another person. A companion is someone who accompanies another person on a journey or for company.

Rival is different from the rest of the options because it is the only word that implies competition. The other words all imply cooperation or companionship.