I'd say that the answer is actually 8 (Option D).
Carcass, your first explanation is correct but the sum of 1+3+3+1 = 8, not 9.
For further expand, according to the ETS Math Conventions: "If all of the elements in A are also in B, then A is a subset of B."
In order for set S to be a subset of M, all of the elements of S must also be in M.
Therefore, we know that M must always contain {2, 4, 6}.
In order for set M to be a subset of T, all of the elements of M must also be in T.
Therefore, we know that any additional elements contained in set M must either be 8, 10, or 12. (It can't be some random number like 13 because 13 is not an element of set T.)
This becomes a combination problem; i.e., how many ways can you create a combination of {8, 10, 12} using either 0, 1, 2, or all 3 of the elements?
When set M contains all of the elements of S and 0/3 other members of T: 1 set- contains {} = {2, 4, 6}
When set M contains all of the elements of S and 1/3 other members of T: 3 sets- contains only {8} = {2, 4, 6, 8}
- contains only {10} = {2, 4, 6, 10}
- contains only {12} = {2, 4, 6, 12}
When set M contains all of the elements of S and 2/3 other members of T: 3 sets- contains {8, 10} = {2, 4, 6, 8, 10}
- contains {10, 12} = {2, 4, 6, 10, 12}
- contains {8, 12} = {2, 4, 6, 8, 12}
When set M contains all of the elements of S and all 3 other members of T: 1 set- contains {8, 10, 12} = {2, 4, 6, 8, 10, 12}
Thus, there are a total of 8 possible sets.