Which device is using a motor?

a water heater creates heat energy from electric energy.

a waterfall rotates a waterwheel to turn an axle.

a body massager uses electric energy to move back and forth.

a solar charger creates electric energy from the sun.

Out of all the options, only body massager which uses electric energy to move back and forth is using motor here. It is because a motor converts electrical energy to Mechanical energy and only massager obeys it among all the options.

In short, Your Answer would be Option C

Hope this helps!

Sorry my friend i don't know the answer. i am just in 6th

What formula  this one  (x_/