The correct answer is B. what.
The sentence “I did what I could which wasn’t much” is an example of a complex sentence. The main clause is “I did what I could”, and the subordinate clause is “which wasn’t much”. The subordinate clause is a relative clause, which is used to modify a noun or pronoun. In this case, the relative clause modifies the pronoun “what”.
The relative pronoun “what” can be used to refer to a noun or pronoun that has already been mentioned. In this case, the noun that “what” refers to is “I”. The relative clause “which wasn’t much” tells us something about “I”. It tells us that “I” could not do much.
The other options are not correct because they do not make sense in the context of the sentence. The relative pronoun “that” can be used to refer to a noun or pronoun that has already been mentioned, but it cannot be used to refer to an action. The relative pronoun “how much” can be used to ask about the amount of something, but it cannot be used to refer to an action. The relative pronoun “which” can be used to refer to a noun or pronoun that has already been mentioned, but it cannot be used to refer to an action.