I am not good with this math either so my guess would be to choose C for all numbers shown are positive
Say that you're thinking of number 
You multiply it by 8, and have 
You add -1 (i.e., you subtract 1) and have 
This equals -17, so you have

Add 1 to both sides

Divide both sides by 8:

So, you were thinking of -2.
The probability of getting no points on any given question is 2/3, so for 20 independent questions, it is (2/3)^20 ≈ 3.007*10^-4.