Acell phone charger uses 4.83 joules per second. when plugged into an outlet but only 1.31 joules per second actually goes into the phone battery

Then there are another (4.83 - 1.31) = 3.52 joules per second (watts) that go
somewhere else.  They become heat that makes the charger warm up while
it's working.

If these figures are accurate, then it's interesting to notice that the device
is almost 3 times as efficient when you call it a heater as it is when you
call it a battery charger.

Ithink it is letter b, i may be wrong but i think that is the answer