Answer :- (C) Brightness of B1 becomes zero after switch is opened.
B1 is directly connected to battery, whereas B2 and B3 are connected through inductors.
When switch is opened current through B1 becomes zero instantaneously.
Wheras rate of decrease of current through B2 and B3 are slow,
because magnetic field generated in the inductor due to change in current opposes the rate of decrease
of current through B2 and B3