Lifting of ball in play

a. is the correct answer. And on the facts of the question, it does not even rely on the 8.2 Exception.

The key to understanding is be aware that the plug mark, once that ball is lifted to take relief, is not part of the CATS when the player is dropping into the relief area - the plug mark is outside the relief area and the player does not have a line of play. Even if the player intends to putt directly over that plug mark and sought to drop to achieve that, it is not an 8.1 issue, but an 8.2 issue, so if the player also intends to care for the course (as in the OP) then the 8.2 Exception applies.

But don't take my word for it, this is published USGA material:

"Your ball is embedded in its own pitch-mark in the fringe just off the putting green. After lifting the ball and before taking relief, you tap down the pitch-mark to care for the course and then realize the repair of the pitch-mark could help you if the ball comes to rest in a position where you might have to play through it. The dropped ball hits and comes to rest in the relief area and you play a low running chip directly over the pitch-mark. You get the general penalty under Rule 8.1a for improving your conditions affecting the stroke. True or false?

Answer is false. Explanation: When and why you repaired the pitch-mark are both critical factors. First, the pitch-mark was not part of the CATS when you repaired it so Rule 8.1 does not apply - it was not part of the relief area (see Rule 16.3a for how embedded ball relief works) and you did not yet have a line of play.
Additionally, Rule 8.2 would apply only to actions you took to deliberately affect where your ball might roll or to affect a future stroke, which wasn't why you fixed it. No penalty and play on."

Here some further Q/As that help elucidate the issues (the questions are mine, the bolded answers are the USGA's):

Question A: your ball is embedded in the fringe just off the putting green. After lifting the ball and before taking relief, you tap down the pitch-mark because your preferred dropping location is precisely behind the pitch-mark and it could interfere with your intended putt. You drop the ball and proceed to putt. What is the ruling? The player is in breach of Rule 8.2

Question B: your ball is embedded in the fringe just off the putting green. After lifting the ball and before taking relief, you tap down the pitch-mark because your preferred dropping location is precisely behind the pitch-mark and it could interfere with your intended putt but also to care for the course. You drop the ball and proceed to putt. What is the ruling? No penalty, the "care for the course exception" in Rule 8.2 applies.