Step 1: Understanding the Theory of Becoming.
The theory of Becoming is most commonly associated with Heraclitus, a pre-Socratic philosopher. Heraclitus famously stated that "everything flows" (Panta Rhei), meaning that the universe is in a constant state of change, and reality is shaped by this continuous flux.
Step 2: Analyzing the Options.
- 1. Plato: Plato believed in the theory of forms, which suggested that the material world was a mere reflection of the perfect, unchanging world of forms. This is different from the concept of Becoming.
- 2. Protagoras: Protagoras is famous for his claim that "man is the measure of all things," but he did not develop the theory of Becoming.
- 3. Heraclitus: This is correct. Heraclitus is known for the theory of Becoming, emphasizing that everything is in constant change.
- 4. Parmenides: Parmenides argued the opposite of Heraclitus, asserting that change is an illusion and that "being" is unchanging.
Step 3: Conclusion.
The correct answer is 3. Heraclitus.
Final Answer:
\[
\boxed{\text{The correct answer is 3. Heraclitus.}}
\]