The correct answer is option 'A' - Slow sand filter.

Explanation:
Slow sand filters are known to produce water of higher bacteriological quality compared to the other filters mentioned.

1. Slow sand filter:
- Slow sand filters are large beds of sand through which water passes slowly.
- These filters operate at a slow flow rate, typically around 0.1 to 0.2 meters per hour.
- The water moves through the sand bed, and suspended particles and microorganisms are trapped in the pores of the sand.
- The trapped particles and microorganisms are then subjected to biological processes, such as predation by microorganisms living in the sand, which further helps in the removal of bacteria and other pathogens.
- The slow flow rate and the biological activity in the filter bed contribute to the high bacteriological quality of the filtered water.
- Slow sand filters are effective in removing bacteria, viruses, and other pathogens, making them suitable for producing safe drinking water.

2. Rapid sand filter:
- Rapid sand filters operate at a higher flow rate compared to slow sand filters.
- These filters typically have a coarser sand media and operate at a flow rate of 2 to 5 meters per hour.
- Rapid sand filters primarily rely on physical processes, such as straining and adsorption, to remove suspended particles from the water.
- While rapid sand filters can effectively remove larger particles, they may not be as effective in removing bacteria and smaller pathogens.
- The higher flow rate and the absence of biological activity in the filter bed may contribute to lower bacteriological quality compared to slow sand filters.

3. Pressure filter:
- Pressure filters are commonly used for water treatment in small-scale systems.
- These filters consist of a pressure vessel filled with a filtration media, such as sand or activated carbon.
- Water is forced through the filter media under pressure, and suspended particles are trapped.
- Pressure filters can be effective in removing suspended particles, but their performance in terms of bacteriological quality depends on the specific design and operation.

4. Dual media filter:
- Dual media filters consist of two layers of filtration media, typically anthracite coal and sand.
- The filter bed is designed to have larger particles at the top and finer particles at the bottom.
- The coarser media layer removes larger particles, while the finer layer provides additional filtration.
- The performance of dual media filters in terms of bacteriological quality can vary depending on the specific design and operation.

In summary, slow sand filters are known for their high bacteriological quality due to their slow flow rate, biological activity in the filter bed, and effective removal of bacteria and pathogens.