The correct answer for the type of reservoir used in a single column manometer is option 'B' - Large.

A manometer is a device used to measure pressure differences between two points in a fluid. It consists of a U-shaped tube partially filled with a liquid, and the pressure difference is determined by the height difference of the liquid columns in the two arms of the tube.

In the case of a single column manometer, there is only one liquid column connected to the system being measured. This means that one end of the manometer is open to the atmosphere, while the other end is connected to the system. The height difference of the liquid column in the manometer indicates the pressure difference between the system and the atmosphere.

To ensure accurate readings, it is important to have a large reservoir connected to the system being measured. This reservoir acts as a buffer, ensuring a constant and steady flow of fluid into the manometer. It helps to minimize any fluctuations in pressure caused by variations in the system.

Having a large reservoir also helps to minimize the effect of any small changes in the fluid level in the manometer. If the reservoir is small, even a slight change in the fluid level can result in significant pressure differences, leading to inaccurate readings.

In contrast, a large reservoir provides a greater volume of fluid, allowing for a more stable and consistent pressure measurement. This reduces the chances of errors and provides reliable readings.

Additionally, a large reservoir also allows for a longer response time, which is beneficial when measuring fluctuating pressures or when dealing with systems that have dynamic pressure changes.

Therefore, it is crucial to use a large reservoir in a single column manometer to ensure accurate and reliable pressure measurements.