The correct answer is option C, 75 cm.

Slow sand filters are a type of water treatment system that use a bed of sand to remove impurities from water. The sand bed acts as a physical filter, trapping suspended solids and allowing clean water to pass through. The maximum filter head refers to the height of the water column above the sand bed.

Here's a detailed explanation of why the maximum filter head for slow sand filters is typically 75 cm:

1. Understanding slow sand filters:
- Slow sand filters are designed to operate at low filtration rates, typically ranging from 0.1 to 0.3 meters per hour.
- The filter bed consists of several layers, including a layer of coarse gravel at the bottom, followed by layers of progressively finer sand.
- The water to be treated is introduced at the top of the filter and percolates through the sand bed, with impurities being trapped in the sand.

2. Importance of filter head:
- The filter head, or the height of the water column above the sand bed, is an important parameter in slow sand filter design.
- It affects the rate of filtration, the pressure drop across the filter, and the efficiency of the filter in removing impurities.
- A higher filter head can increase the filtration rate, but it also increases the pressure drop and the risk of the filter clogging.

3. Maximum filter head for slow sand filters:
- The maximum filter head for slow sand filters is typically limited to around 75 cm.
- This limit is based on practical considerations and the need to maintain efficient filtration.
- If the filter head exceeds 75 cm, the pressure drop across the filter increases significantly, making it difficult for water to pass through the sand bed.
- Higher filter heads also increase the risk of clogging, as the sand particles may become packed together too tightly, reducing the pore space for water flow.

4. Factors affecting filter head:
- The maximum filter head for slow sand filters can vary depending on various factors, including the size and shape of the sand particles, the depth of the sand bed, and the water quality.
- Finer sand particles tend to have a lower maximum filter head, as they have smaller pore spaces and are more prone to clogging.
- Deeper sand beds can tolerate higher filter heads, as the weight of the water helps to maintain efficient filtration.

In conclusion, the maximum filter head for slow sand filters is typically limited to around 75 cm. This limit ensures efficient filtration and prevents excessive pressure drop and clogging.