The term "indispensable" refers to something that is absolutely necessary or essential, without which something cannot function. Here's the analysis of the options:
- Impracticable: Refers to something that is not feasible or workable, the opposite of "indispensable."
- Impossible: Refers to something that cannot be done at all, which is not the same as "indispensable."
- Unmanageable: Refers to something that cannot be managed, unrelated to the concept of something being essential.
- Indispensable: Means something that is absolutely necessary, which is the correct meaning of "that which cannot be done without."
Therefore, the correct answer is "indispensable".