Which of Newton's laws states that the rate of change of a body's momentum is proportional to the force applied to it?

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The rate at which momentum changes is governed by the net external force acting on an object. Momentum is defined as p = m v, so the rate of change is dp/dt. Newton's second law tells us that this rate of change equals the applied force: F = dp/dt. If the mass stays constant, this reduces to F = m a, showing that force directly controls acceleration and thus how quickly momentum changes. For example, pushing harder on a cart increases the acceleration and increases the speed at which its momentum grows. If the mass isn't constant, the relation still holds in the form F = d(mv)/dt, though you must account for changing mass as well. The other Newton’s laws describe different ideas: inertia for objects in motion or at rest in the absence of net force, and action-reaction pairs, while the law of gravitation explains the gravitational attraction between masses.