Option A: rolling a prime number:
2 can be rolled only 1 way (1,1)
3 can be rolled two ways (1,2) or (2,1)
5 can be rolled four ways (1,4) or (2,3) or (3,2) or (4,1)
7 can be rolled six ways (1,6) or (2,5) or (3,4) or (4,3) or (5,2) or (6,1)
11 can be rolled two ways (5,6) or (6,5)
There are 36 possible ways to roll two dice.
The probability of rolling a prime number is: (1 + 2 + 4 + 6 + 2) / 36 = 15/36
Since rolling a prime number is worth 15 points, the expected value is (15/36) x 15 = 6.25
Option B: rolling a multiple of 3:
3 can be rolled two ways (1,2) or (2,10
6 can be rolled five ways (1,5) or (2,4) or (3,3) or (4,2) or (5,1)
9 can be rolled four ways (3,6) or (4,5) or (5,4) or (6,3)
12 can be rolled only 1 way (6,6)
The probability of rolling a multiple of 3 is: (3 + 6 + 9 + 12) / 36 = 12/36
Since rolling a multiple of 3 is worth 12 points, the expected value is (12/36) x 12 = 4
If you wish to maximize your expected point total choose Option A.
[I don't understand some of your expanation.]