Which of the following metal will pollute water?

The correct option is Cd.

To understand why cadmium is a water pollutant, we can look at the characteristics and biological roles of the metals listed in the options:
1. Cadmium (Cd): Cadmium is a heavy metal that is highly toxic even in trace amounts. It enters water bodies primarily through industrial discharges, electroplating waste, mining operations, and runoffs from agricultural fertilizers containing phosphates. Since it is a non-biodegradable heavy metal, it accumulates in the aquatic food chain (bioaccumulation) and poses severe health hazards. Ingestion of cadmium-polluted water can lead to chronic toxicity, kidney damage, and the painful bone-softening condition known as Itai-Itai disease.
2. Sodium (Na) and Potassium (K): Sodium and potassium are essential alkali metals that play crucial physiological roles in human and animal systems. They are necessary for nerve impulse transmission, fluid balance, and muscle function, and naturally occur in healthy aquatic systems. They are not considered toxic water pollutants under standard environmental guidelines.

Therefore, among the given choices, cadmium (Cd) is the metal that pollutes water.