Which of the following is an indicator of a tsunami?

The phenomenon where water recedes significantly from the coastline is a classic indicator of an impending tsunami. This occurs as the tsunami approaches the shore; the ocean pulls back to allow for the massive wave that is about to surge in. This receding effect can happen suddenly and dramatically, exposing large areas of the sea floor, which is unusual compared to normal tidal fluctuations. It is often accompanied by other indications, such as a noticeable draw of water back from the beach. Observing such a change in water level should prompt immediate action to evacuate to higher ground, as it serves as a critical warning sign of the potential danger that follows.

The other choices do not serve as indicators of an impending tsunami. Thunderstorms and excessive rain clouds are common weather phenomena that do not directly relate to tsunami activity. Likewise, peaceful waves at the shore do not signal the arrival of a tsunami; rather, they indicate typical ocean behavior. Understanding these indicators is vital for disaster preparedness and response.