Which of the following is a major threat to coral reefs?

Coral bleaching due to temperature changes is indeed a major threat to coral reefs. This phenomenon occurs when elevated sea temperatures stress the coral, causing them to expel the symbiotic algae (zooxanthellae) that live in their tissues. These algae provide the corals with food through photosynthesis and give them their vibrant colors. Without these algae, corals not only lose their color but also become more vulnerable to disease and can starve if the stress continues for an extended period. Prolonged bleaching can lead to significant declines in coral populations, ultimately disrupting the marine ecosystems that rely on healthy coral reefs as a habitat.

Natural volcanic activity, while impactful, typically occurs over geological timescales and is not a consistent threat to coral reefs compared to the immediate and widespread effects of climate change. Excessive sunlight can harm corals but is usually not a factor on its own, as corals are adapted to their specific light conditions. Likewise, while coral polyps do multiply, overpopulation is not a commonly recognized threat to coral reefs, as healthy reef systems support robust populations of coral without significant issues. Therefore, the immediate and severe impact of temperature changes leading to coral bleaching positions it as a critical concern for the health of coral reefs worldwide