Mobilization of metals in acid water becomes a direct threat to human health when:

The mobilization of metals in acid water becomes a direct threat to human health primarily when the acidified water is a source of drinking water. In such scenarios, the mobilization of metals like lead, mercury, and cadmium is heightened due to lower pH levels, allowing these toxic substances to leach from sediments and mineral formations into the water. When humans consume this water, they are at risk of heavy metal poisoning, which can lead to a range of health issues, including neurological and developmental problems, organ damage, and various chronic health conditions.

While the presence of plankton, fish, or algae can indicate ecological changes or disruptions in aquatic ecosystems, they do not pose the same immediate health risk to humans as drinking contaminated water. Additionally, the mere death of fish or changes in algal populations may reflect environmental impacts but do not directly connect to human health risks without the context of human consumption of the affected water. Therefore, the correct answer emphasizes the pathway through which pollutants lead to potential health effects on people.