Answer: C. to have been disqualified
Step-by-step explanation:
Some athletes are believed to have been disqualified from yesterday's race due to their having taken drugs.
A present perfect tense would fit here best because it shows actions that were continuing in the present even though they started in the past.
The disqualification was in the past in this scenario, but still continues in the present. "Have been" is a present perfect clause used when the subject refers to a group of people as it does here so it is best used here.