Question

A person applies a rightward force of 80 N to a cart of physics textbooks what is the reaction force

Answer 1

Just as Prof. Newton said: "For every action, there is an EQUAL, OPPOSITE reaction.". (See his motion law #3.)

If our person APPLIED a force of 80N pointing to the right, then that's the 'action'.  It must follow, as the night follows the day, that the strength of the equal REaction is also 80N, and it points to the left because it's opposite to the action.