The correct answer is: A. Excessive negative head.
Air binding phenomena in rapid sand filters occur due to excessive negative head. This is because when the head pressure on the filter is too low, air can be drawn into the filter bed. This can cause the filter to become clogged and inefficient.
Mud ball formation, higher turbidity in the effluent, and low temperature can all contribute to air binding, but they are not the primary cause.
Mud ball formation can occur when there is a lot of organic matter in the water. This organic matter can form clumps, which can then block the pores in the filter bed. This can prevent water from flowing through the filter and can also lead to air binding.
Higher turbidity in the effluent can also contribute to air binding. This is because when there is a lot of suspended solids in the water, these solids can block the pores in the filter bed. This can prevent water from flowing through the filter and can also lead to air binding.
Low temperature can also contribute to air binding. This is because when the temperature is low, the water molecules are less likely to move. This can make it more difficult for water to flow through the filter bed and can also lead to air binding.
However, these factors are not the primary cause of air binding. The primary cause of air binding is excessive negative head.