According to the teachings of the Buddhist and Hindu religions, attaining nirvana is to enter a state of supreme liberation, leaving behind the desires that perpetuate the cycle of death and rebirth.

(A) attaining nirvana is to enter
(B) to attain nirvana is entering
(C) to attain nirvana is
(D) to attain nirvana is to enter
(E) attaining nirvana is to be entering

(F) attaining nirvana is entering

[spoiler]OA: D[/spoiler]
I was able to mark the correct answer on the basis of parallelism, but can someone please explain what is the modifier in the sentence and its role.

Also, whether the additional option F is correct or not?

Thanks