That which cannot be done without

A. Irrevocable

B. Impracticable

C. Indispensable

D. Impossible

Answer: Option C

Solution(By Examveda Team)

One word substitute is Indispensable.

Irrevocable: not able to be changed, reversed, or recovered; final.
Impracticable: (of a course of action) impossible in practice to do or carry out.
Indispensable: absolutely necessary.
Impossible: not able to occur, exist, or be done.