Answer
(a) less than
(b) II
Work Step by Step
We know that $F_A=F_2-F_1=0$
and $F_B=\sqrt{F_1^2+F_2^2+2F_1F_2cos\theta}\gt 0$
$\implies F_B\gt F_A$
If the change at A is positive, then the resultant force due to charges on either side is $F_A=0$. The net force experienced by the positive charge at point B is greater than zero: $F_B\gt F_A$.
(b) We know that the best explanation is option (II) -- that is, the net force at point A cancels. Therefore, the nonzero net force at point B is greater in magnitude than the zero net force at point A.