In the context of Newton's First and Second Laws, which statement best defines an unbalanced force acting on an object?

• A. A force that always points in the direction of motion and has magnitude $F_{\mathrm{net}}=mv$.
• B. A situation where all forces are equal in magnitude and opposite in direction so the net force is $F_{\mathrm{net}}=0$, causing constant velocity.
• C. Any force acting on an object that is moving at a constant speed in a straight line.
• D. A net force not equal to zero, meaning the vector sum of all forces on the object is $F_{\mathrm{net}}\ne 0$, which can cause acceleration.

Answer: (D)