a) Darwin's theory explains the formation of coral reefs, which are the underwater rock-like structures, through a process of subsidence. It proposes that reefs begin as fringing reefs growing around a volcanic island. As the island slowly sinks (subsides), the coral continues to grow upwards towards the sunlight, forming a barrier reef separated from the island by a lagoon. With continued subsidence, the island eventually disappears below the surface, leaving a ring-shaped atoll enclosing a central lagoon. b) These underwater rock-like structures (coral reefs) are crucial to the people of East Africa for several reasons. They are biodiversity hotspots, providing essential habitats for a vast array of marine life, including fish, which are a primary source of food and income for local communities through fisheries. The reefs also support a significant tourism industry, attracting divers and snorkelers, thereby creating jobs and economic opportunities. Additionally, they serve as natural coastal protection, buffering shorelines from erosion, storm surges, and strong waves.