The correct answer is:
B) 7.
Step-by-step explanation:
A set is a subset of itself. This is 1 subset.
Each individual letter can be taken as a subset; this gives us {A}, {B}, and {C}. This is 3 more subsets for 4 so far.
{A,B}, {A,C}, and {B,C} are also subsets. This makes 3 more, for a total of 7