Which of the following forces causes comets to regularly return to the inner solar system after being gone for many years?

Unfortunately, you neglected to include the list of allowable choices that "follows"

the question. Still, I know the answer.

The force that keeps comets in closed orbits around the sun is the one that has

the same effect on the planets; gravity. The only difference is that the orbits

of the long-term comets are much more 'eccentric' (long and skinny) than those

of the planets, so they have much shorter minimum distances from the sun, and

much longer maximum distances, than planetary orbits have.

So, the correct awnser is Gravity.