The correct option is 1, 2, and 3 because ocean acidification primarily results from increased carbon dioxide absorption, which lowers the pH of seawater. This process adversely affects calcareous organisms, including phytoplankton and coral reefs, that rely on carbonate ions to build their calcium carbonate structures. Additionally, many marine animals, particularly those with larval stages that depend on phytoplankton, may also face survival challenges due to the decline in phytoplankton populations, disrupting marine food webs and ecosystems. These interconnected impacts highlight the far-reaching consequences of acidification on marine biodiversity and ecological stability, essential topics in environmental science for UPSC examinations.