That which cannot be done without

The correct answer is C. Indispensable.

Something that is indispensable is essential or absolutely necessary. It is something that cannot be done without or that is absolutely essential to the success of something else.

The other options are incorrect because they do not mean the same thing as indispensable.

• Irrevocable means that something cannot be changed or taken back.
• Impracticable means that something is not possible or feasible.
• Impossible means that something cannot be done or achieved.

Here are some examples of indispensable things:

• Oxygen is indispensable for life.
• A good education is indispensable for success in today’s world.
• A strong foundation is indispensable for a well-built house.
• A good team is indispensable for winning a championship.

I hope this helps! Let me know if you have any other questions.