Vector 'a' is made out of some 'up' and some 'right'.
Vector 'b' cancels most or all of the 'up' part of 'a', leaving only the 'right' part and maybe a slight bit of 'up'.
The only choice that is mostly 'right' with maybe a slight bit of 'up' is choice-B.
Choice-A has too much 'up' in it.
Choices C and D both have some 'left' in them. They couldn't get any 'left' from 'a' or 'b'.