Among the options provided, melting mountain glaciers have the greatest effect on drinking water supplies. Many regions rely on glacial meltwater as a critical source of freshwater, especially in areas where other water sources may be limited. As glaciers melt due to global warming, there may initially be an increase in water flow, but over time, the reduction in glacial mass can lead to decreased water availability during dry seasons, ultimately threatening drinking water supplies.
Rising sea levels primarily affect coastal areas, potentially leading to saltwater intrusion into freshwater aquifers, but the immediate impact on drinking water supplies is less direct compared to the effects of melting glaciers. Early spring blooming of flowers is a sign of ecological change but does not have a direct impact on drinking water supplies.