Newtonh third law of motion describes a net force. b action and reaction forces. c. balanced forces. d centripetal forces.

## Step 1<br />The problem is asking us to identify what Newton's third law of motion describes. Newton's third law of motion states that for every action, there is an equal and opposite reaction. This means that any force exerted on a body will create a force of equal magnitude but in the opposite direction on the object that exerted the first force.<br /><br />## Step 2<br />Let's analyze each option:<br />- Option A: A net force is the vector sum of all forces acting on an object. This is not what Newton's third law describes.<br />- Option B: Action and reaction forces are forces that are equal in magnitude and opposite in direction. This is exactly what Newton's third law describes.<br />- Option C: Balanced forces are forces that are equal in magnitude and opposite in direction, but they do not cause a change in motion. This is not what Newton's third law describes.<br />- Option D: Centripetal forces are forces that keep an object moving in a circular path. This is not what Newton's third law describes.