Answer
(a) $-Q$
(b) $+Q$
(c) 0
Work Step by Step
(a) We know that the net charge on D is $+Q$. We can see from the first set that the net charge of sphere $A=-Q$ because in this set, both balls are attracted to each other, which shows that they have unlike charges.
(b) We see in the 4th set, the net charge of sphere $B=+Q$. This is because in this set, both balls are repelled to each other, which shows that they have like charges.
(c) We know that in the 3rd set, the net charge of sphere $C=0$ because in this set, both balls are weakly attracted to each other due to polarization. This shows that C has no charge.
