Step 1: Sequence of Pre-Socratic Philosophers.
Understanding the chronological order of Pre-Socratic philosophers is vital for grasping the evolution of early Greek philosophy. - Thales (A) is widely recognized as the inaugural Western philosopher, notably for his theory that water constitutes the universe's fundamental element. - Anaximenes (B) succeeded Thales, positing air as the primary substance of the cosmos, superseding water. - Anaximander (C), who followed Anaximenes, introduced the "Apeiron" (the infinite or boundless) as the primordial source of all existence. - Pythagoras (D) emerged later, renowned for his mathematical advancements and the principle of number as the core of reality.
Step 2: Conclusion. The correct chronological sequence of these philosophers' contributions is A, B, C, D.
Final Answer: \[ \boxed{\text{The correct answer is 2. A, B, C, D.}} \]