Correct answer is option C .
Lets say A has x no of marble and B has y no of marble.
now A says,
If you give me one marble then I will have twice as many as you will have, which means
$x+1=2(y-1)$
or, $x-2y=-3$ ….....(1)
Now B says,
If you give me one marble then I will have as many as you will have , which means
$x-1=y+1$
or,$x-y=2$ …………(2)
(2)-(1)
y=5,
so , x=7
so x+y=12.
correct answer is option c