Which theory states that all matter is composed of smaller particles called protons, electrons, and neutrons?

The correct answer, which states that all matter is composed of smaller particles called protons, electrons, and neutrons, is grounded in Atomic Theory. Atomic Theory is a fundamental principle in chemistry and physics that provides the framework to understand the composition and structure of matter.

This theory posits that matter is made up of atoms, which consist of a nucleus containing protons and neutrons, with electrons orbiting around this nucleus. Protons carry a positive charge, neutrons have no charge, and electrons carry a negative charge. The interplay between these particles gives rise to the various properties of different elements and compounds.

While other theories, like Quantum Theory, focus on the behavior of particles at very small scales, Atomic Theory is specifically concerned with the structure of matter itself, making it the most accurate choice in this context. Electron Theory primarily deals with the behavior and properties of electrons, while Nuclear Theory focuses on the components and interactions at the nucleus level. Hence, none of those options comprehensively addresses the entirety of matter's composition as Atomic Theory does.