Final answer:
The correct phrase to use is 'doesn't matter' because 'matter' is a stative verb that is typically not used in the continuous form.
Step-by-step explanation:
The correct answer is a. doesn't matter. In this context, we are referring to a fact or state (it doesn't matter), not a continuous action, so the simple present tense is appropriate. The phrase "isn't mattering" is not used because "matter" is a stative verb that does not usually use the continuous form. Instead, we use the simple present tense for states and facts that are generally true or not dependent on time. Therefore, the sentence should read: "It doesn't matter which of the books we've studied this term you do your essay on."