The number of ways of choosing 3 objects from 6 objects is 6C3.
6C3 = 6!/[3!(6-3)!] = 6!/(3!3!) = 6*5*4/(3*2*1) = 20
This is the number of outcomes in the sample space. One of those outcomes will yield her 3 favorite sweaters.
So, the probability is 1/6C3 = 1/20
Therefore, the answer is Option 3.