Concept: An irrational number cannot be written as a simple fraction of two integers. Explanation: π ≈ 3.1415926535... has a non‑terminating, non‑repeating decimal expansion. It cannot be expressed in the form qp where p and q are integers and q=0. Therefore, π is not a rational number, nor a fraction (proper or improper), and not a prime number (since prime numbers are positive integers greater than 1). Answer: D. An irrational number