John proctor admits to having an affair with abigail only when

a. reverend hale suspects that he had an affair with her
b. abigail threatens to expose the truth to the court
c. he realizes that he may be able to spare elizabeth from a death sentence
d. all of the above

I believe the answer is C

D.all of the above

Explanation:

John Proctor lies about having an affair because he believed that he could get out of jail and sentence without having to do tell the truth, inly untill it is too late is that he realizes that he is already a dead man walking and that his honor is already lost, the last thing he does is try to save his wife from the death sentence by confessing, the reverend Jale already suspected that and Abigail had threatened to expose the truth, it´s all of this and the clear sight of death that make him confess the infidelity but it was already too late.

explanation:

it refers to the definition of full-contact sports, where the condition is that the sport must have athletes must "purposely collide with each other