In a parallel circuit, if one of the bulbs blew out, the other bulb would
a. not be able to light up.
b. stay lit but be very dim.
c. stay lit in the same way.
d. stay lit and be twice as bright as when both bulbs were lit.

In a parallel circuit, both bulbs are connected directly to the power supply
or battery.  As long as the power supply is capable of running both of them,
neither bulb is aware of the other's presence.  If one bulb fails 'open', the
other bulb doesn't change at all.  The voltage across it, the current through it,
its resistance, and its brightness,all remain exactly as they were before.