Question

A dog pulls on his leash with a 10-n force to the left, but doesn't move. identify the reaction force.

Answer 1

some thing or some one it use a using a opposite force

Answer 2

The reaction force is the tension in the leash.

It must be 10N to the right. Otherwise, the forces on the dog would be
unbalanced, the net force on the dog would not be zero, and the dog
would be accelerated either toward the left or toward the right.