ESSENTIAL :

Desirable
Advisable
Beneficial
Unavoidable

The correct answer is D. Unavoidable.

Essential means something that is absolutely necessary or very important. Desirable means something that is wanted or wished for. Advisable means something that is recommended or suggested. Beneficial means something that is helpful or advantageous.

Of the four options, only unavoidable is absolutely necessary. Something that is desirable, advisable, or beneficial may be nice to have, but it is not essential.