For stem 1:
We can think of Two scenarios:
First scenario: S=(-3,-1,1,3,5). Here the mean = 5/5 = 1. Median =1
Second scenarion: S =(-3,-1,1,3,5,7). Here the mean = 12/6 = 2. Median =(1+3)/2 = 2. Same results. We can conclude that the avg = median. A firm Yes.
Thus, sufficient.
For stem 2:
Its given that the range is 2(n-1). Lets consider a 4 member set. We will then have the range as 2(4-1) =6. The set can be S= (1,0,0,7) which has Range = 6. The avg is not equal to Median here. The set can also be (0,3,3,6) again the range being 6. However, here, the avg = median. We cannot conclude that avg = median for sure with this stem. Thus Insufficient.
The answer is A.