I will attempt a.
For the last 12 positions there are ways to pick the 0's, once the 0's are fixed, the 1's fall naturally into position.
Then for the first 15 positions there are 2 choices for each positions, either a 1 or 0, thus there are choices for the 1st 15 positions.
Thus in total we have bit strings.
I would really like to know if this is correct though, could someone confirm? Thanks.