That which cannot be done without
A
Irrevocable
B
Impracticable
C
Indispensable
D
Impossible
Correct Answer: Indispensable
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.