recognize strings of 1's and 0's which are multiples of 3
If the question intends anything other than your option&nbsp;2 (binary representations of natural numbers that are divisible by&nbsp;3), then it is a badly written question.

If the qeustion meant strings in which the number of 0s and the number of 1s are both divisible by&nbsp;3, it would have said that. It would be bizarre to have numbers whose digits are all 0s and 1s and intend some base other than binary without mentioning that.