99% of the time I would say to plug in on questions like this, but it's more of a pain than it's worth here. The numbers are just too big and the answer choices require too many calculations. Instead, use algebra:

If the first three sandwiches were divided equally, each person ate 1/m of the sandwich.

For the fourth sandwich, four of the students dropped out, so now we only have m-4 students. So, each ate 1/(m-4) of the sandwich.

Thus, Carol ate 1/m + 1/m + 1/m + 1/(m-4) of the sandwiches. Still a bit of a pain to work out that algebra, but it's faster than plugging in on this particular problem.