One formula for the power dissipated in an electric circuit is
Power = (current)² x (resistance) .
I think that's the one we should use, because it has exactly
all of the quantities that we know or need to find.
OK.
Original power = (original current)² x (original resistance)
and
New power = (new current)² x (new resistance)
= (0.5 x old current)² x (3 x old resistance)
= (0.25) (old current)² x (3) (old resistance)
= (0.75) (original current)² (original resistance)
= (0.75) (original power) .
Summary: New power = (0.75) (original power)
That looks just like
choice B:
The power will decrease to 75 percent of its previous value.
