Answer : 1. "a wave reflects upon itself"
Explanation :

Answer: A) a wave reflects upon itself Explanation: A standing wave also known as a stationary wave, is a wave in which its peaks or any other point on the wave do not move spatially. This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling in opposite directions. The most common cause of standing waves is the phenomenon of resonance, in which standing waves occur inside a resonator due to interference between waves reflected back and forth at the resonator's resonant frequency.   Hence, standing wave occurs when a wave reflects upon itself.