Explain why an electric heater is the only appliance that can possibly be 100% efficient.?

All other appliances have heat losses produced as a side effect of whatever they are doing. Only the intended work / function is 'counted' when the efficiency is worked out.


A heater may also has heat 'losses' in the wiring etc. however all those 'losses' are 'counted' as part of the intended function (which is to give out heat)

It has no moving parts that do work, so almost all the electric energy gets converted into heat by electrical resistance.

If you hate physics then probably you have an uninspiring teacher. Don't blame physics...