Read the sentence.

Completing the task, which is taking more than four hours, and is a grueling endurance test that only a few people have successfully finished.

Which is the best way to rewrite the sentence?

a. The task, taking more than four hours to complete, which is a grueling endurance test that only a few people have successfully finished.
b. Taking more than four hours to complete, the task is a grueling endurance test that only a few people have successfully finished.
c. Completing the task is a grueling endurance test that only a few people have successfully finished, which is taking more than four hours.
d. That which is taking more than four hours to complete, the task is a grueling endurance test that only a few people have successfully finished.