McQ's

The shaft. To the frame, to the axle to the wheels.

ANSWER a.
Action is the force which initiates movement or deformation.
The key word is "when" with respect to initiation. With or without the horse the weight of the cart could be deemed an "action" but that ignores the "when" in your context.

Ahhhh ..... what do you think I said?????

I see the reason I put you on ignore originally. (Subject to occasional review like this one...alas no change yet).

You're an idiot. Ignore me. But I won't ignore you.