Defined as how much energy it takes to perform a specific amount of work.

a.) machine
b.) friction
c.) efficiency
d.) mechanical advantage

Defined as how much energy it takes to perform a specific amount of work.

B. Friction

I think the best answer from the choices listed above is option B. Friction force is defined as how much energy it takes to perform a specific amount of work since it is the only force among the choices listed above. Hope this answers the question.

If there was no gravity then their would be no downword force on the football to have it return the the grownd, and if there was no friction (mainly air friction) then there would be no opposing force the stop the ball from moving through the air allowing the ball to travle ∞.