Debbie places two shopping carts in a cart corral. She pushes the first cart, which then pushes a second cart. What force is being exerted? Debbie exerts force on the first shopping cart. The first cart exerts force on the second cart.

Yes, debbie pushes on the 1st cart, but the 1st cart also pushes back on debbie with equal and opposite force, same with the 2nd cart on the 1st cart. Debbie and the 2nd cart don't directly interact, but there is a force reaction pair between the two carts and between debbie and the first cart. We also have implied normal forces that cancel with weight forces and potentially friction from debbie's shoes on the ground when she pushes and friction on the wheels of the carts. This might be overkill, but the question seems to want you to focus on Newton's 3rd Law, which is that every action has an equal and opposite reaction, those force reaction pairs I talked about. When debbie pushes on the cart, the cart pushes back on her with equal and opposite force.