Nowadays most british children (over 80 percent) often go to comprehensive schools which take pupils of all … without any exams.
(a) ability
(b) abilities
Which one is correct?
You have to ask yourself the question:
do all children have the same type and level of ability or not?
The answer to that should help you decide on the answer you need.
No, not all children have the same type and level of ability, that’s why ----> “abilities”. Am I right?
You’ve got it. 
I reckon, “you’ve got it” is an informal way of saying “You understood”, isn’t it?
That’s right.