What does "Mutational Variance" mean?
In Pigliucci (2004) (repeated in Hansen (2006)) the mutational variance is defined as being the new additive genetic variance entering in the population each generation due to mutations.

In consequence the mutational variance is not define at the level of the mutation in the sense that we do not care about how many mutations yield to this new genetic variance. So none of the suggestions of possible answers in the OP's question is correct.