In what form does the energy wasted because of friction mainly appear?

Think it through first, then pick an option.

Answer and explanation

Correct answer: b heat

Friction produces heat (hands rub warm, engines heat up), so the wasted energy appears as heat.

Why the other options are wrong

a light: Friction mainly produces heat; light is not its main form.
c sound: Some sound may occur, but the book names heat as the form of wasted energy.
d force: Force is not a form of energy; force and energy are different quantities.