The athletes who . . . . . . the games are called competitors. A. enter B. entered C. while entering D. are

The correct answer is: D. are

The athletes who are competing in the games are called competitors.

• Enter means to go or come in, especially to a place where a competition or other event is taking place.
• Entered is the past tense and past participle of the verb enter.
• While entering means during the process of entering.

In this case, the athletes are already competing in the games, so the correct verb to use is are.