I'll go with answer "b" but I think it's a really bad question.
There are a number of ways that gravitational acceleration and net acceleration can be stated. Any of these answers could be correct. But it would depend on whether you defined gravity as exerting a force in a direction along the negative coordinate.
It's a bad question for another reason. It's not really clear what physical insight the student is being asked to demonstrate. Answers "a" and "c" are algebraically equivalent. And answer "b" is clearly different. The answer obviously cannot be "d" which leaves the only unique solution as "b."
The core of the physics that is being probed is the idea that one does not experience any weight while in freefall. Only astronauts experience this for any extended time. Astronauts train for this on the "reduced gravity aircraft" also know as the Vomit Comet. The Vomit Comet can simulate the weightlessness of being in orbit by plunging toward the earth in freefall for about 30 seconds at a time. The passenger's actual acceleration is equal to the gravitational acceleration.
If this question were asked in a context where the acceleration of gravity and all coordinate systems had been well defined, then it might be reasonable. Out of context, it's pretty vague. Some textbooks will use