What theory states that particles are in constant motion, with varying speeds depending on the state of matter?

The correct choice is informed by the understanding of the kinetic theory of matter, which posits that all matter is composed of tiny particles that are in perpetual motion. This motion is influenced by the temperature and energy of the particles, which leads to different behaviors and states of matter—solid, liquid, and gas.

For example, in solids, particles vibrate in fixed positions, leading to a definite shape and volume. In liquids, particles move more freely, allowing the liquid to take the shape of its container while maintaining a definite volume. In gases, particles are widely spaced and move rapidly and freely, resulting in neither fixed shape nor volume. Thus, kinetic theory effectively explains how temperature changes lead to variations in particle motion and the resultant properties of substances in different states of matter.

The other theories presented, while relevant to the study of matter, do not specifically address the constant motion of particles. Particle theory generally refers to the composition of matter, molecular theory often deals with interactions at the molecular level, and atomic theory focuses on atoms as fundamental units, rather than the dynamic behavior of particles in different states.