The most stable ecosystem among the given options is the ocean. Although all ecosystems experience change and fluctuations, ocean ecosystems tend to have greater stability compared to others. This is because the vast size and depth of the ocean provide a buffer against sudden changes in environmental conditions. The constant movement of ocean currents helps distribute nutrients and maintain a relatively stable temperature, promoting a balanced ecosystem. Additionally, the high biodiversity of marine life helps to maintain the stability of the food web. While other ecosystems like deserts, mountains, and forests may have their own unique characteristics and abilities to adapt to change, the ocean`s size, depth, and the interconnectedness of its organisms contribute to its overall stability.