GMAT Sentence Correction (SC) | Re: The learning process often requires

The learning process often requires the repetition of a specific action again and again until it is mastered.

a, the repetition of a specific action again and again until it is
b, specific actions until they are
c, the repetition of a specific action until that action has been
d, the repetition of a specific action until it is
e, repeating a specific action again and again until it is