Final answer:
The theory that aims to unify all four fundamental forces, including gravity, is called the Theory of Everything (TOE). The Grand Unified Theory (GUT) seeks to unify the strong, weak, and electromagnetic forces, excluding gravity.
Step-by-step explanation:
The theory that aims to unify all four fundamental forces is known as the Theory of Everything (TOE), which is option b in the provided question. The TOE seeks to combine the strong nuclear force, weak nuclear force, electromagnetic force, and gravitational force into a single theoretical framework. In contrast, the Grand Unified Theory (GUT) focuses on unifying only strong, weak, and electromagnetic forces, excluding gravity. It attempts to explain the interactions at high energy levels where these forces appear to be different manifestations of a single force and also acknowledges why this unification breaks down at lower energy scales. While there has been success with the GUT, particularly in showing that the electromagnetic and weak nuclear forces are indistinguishable under certain high-energy conditions (the electroweak force), the TOE, which would include gravity, remains more elusive due to the challenge of integrating the special characteristics of gravity with the other three forces.