**Core Concept**
Biological (slow sand) filters are a type of water treatment process used to remove suspended solids and contaminants from water. The flow rate of water through the filter is a critical parameter that affects the effectiveness of the treatment process. Venturi meters are often used to measure the flow rate of water in such systems.

**Why the Correct Answer is Right**
A Venturi meter is a device used to measure the flow rate of fluids in a pipe. In the context of a biological filter, it is used to measure the flow rate of water entering the filter. This is crucial in ensuring that the filter operates within its optimal flow range, where the biological activity can effectively remove contaminants from the water. By measuring the flow rate, operators can adjust the system to maintain the desired flow conditions, which helps to prevent clogging and maintain the filter's effectiveness.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Venturi meters are not typically used to measure pressure in a biological filter. While pressure is an important parameter in water treatment systems, Venturi meters are specifically designed to measure flow rate.
* **Option B:** This option is incorrect because there is no direct relationship between a Venturi meter and the removal of contaminants by biological activity. While the flow rate measured by a Venturi meter can affect the filter's effectiveness, the meter itself does not directly measure contaminant removal.
* **Option C:** This option is incorrect because the flow rate measured by a Venturi meter is not directly related to the filter's headloss. While headloss is an important parameter in water treatment systems, it is typically measured using different types of equipment, such as orifices or weir boxes.

**Clinical Pearl / High-Yield Fact**
When operating a biological filter, it is essential to maintain a consistent flow rate to ensure optimal biological activity and contaminant removal. A flow rate that is too high can lead to clogging and reduced filter effectiveness, while a flow rate that is too low can lead to reduced biological activity and ineffective contaminant removal.

**Correct Answer:** C. Flow rate.