Newton's second law states that acceleration is proportional to what?

Newton's second law of motion states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This fundamental principle can be mathematically expressed as F = ma, where F represents the resultant force, m denotes the mass, and a indicates the acceleration.

In this context, the correct choice connects directly with the law's assertion that the acceleration experienced by an object arises from the resultant force acting on it. The greater the resultant force applied, the greater the resulting acceleration, assuming the mass remains constant. This principle is crucial in understanding how varying forces influence an object’s motion.

The options that reference mass, distance, and speed relate to different aspects of motion but do not capture the essence of the relationship described by Newton's second law. While mass is a component of the formula, it is not what acceleration is proportional to. Distance traveled and speed are outcomes of acceleration rather than factors that influence it directly under Newtonian mechanics.