What is the right answer?
I made him ___ the work
The right answer is (a) do.
I made him do the work.
To make somebody/someone do something (force someone do something they do not want to do) is a stable expression in English. Thus, we have to use DO after MAKE somebody.
To remember it easier, use this structure:
MAKE + OBJECT + BARE INFINITIVE*
She made me wash the dishes.
He made her clean his room again.
* Bare infinitive = infinitive without to