The correct answer is option 1: Groundwater. Groundwater is likely to be contaminated with fluoride. Here is a breakdown of each option for better understanding:

Option 1: Groundwater

Groundwater is water that is found underground in the cracks and spaces in soil, sand, and rock. It is considered to be one of the main sources of drinking water for many people around the world. However, depending on the geological composition of the area, groundwater can become contaminated with various chemicals, including fluoride. This usually occurs when groundwater comes into contact with rocks or minerals that contain high levels of fluoride, which then dissolves into the water.

Option 2: River water

River water comes from surface water sources and is constantly moving. While it can become contaminated with various pollutants, fluoride contamination is less common in river water compared to groundwater, as the concentration of fluoride in river water is generally lower.

Option 3: Pond water

Pond water refers to water that is present in small, stagnant bodies of water, usually found in natural depressions or artificially created ponds. Like river water, pond water is less likely to be contaminated with fluoride compared to groundwater, though it can still be contaminated with other pollutants depending on surrounding activities.

Option 4: Rainwater

Rain