ANS:D - 6

The minimum diameter of the tube used for making a water manometer should be 6 mm to avoid correction for the effect of capillarity. Explanation: Capillarity is the phenomenon in which liquids in narrow tubes (capillaries) rise or fall due to surface tension effects. In the context of manometers, capillarity can cause the liquid column to deviate from the true pressure measurement, leading to inaccuracies in the readings. To minimize the effect of capillarity in a water manometer, a larger diameter tube is preferred because capillary action is more pronounced in narrower tubes. By using a tube with a larger diameter, the influence of capillarity on the measurement can be minimized, reducing the need for correction factors. A diameter of 6 mm or larger is typically considered sufficient to minimize the effect of capillarity in water manometers, allowing for more accurate pressure measurements without the need for correction. Therefore, a minimum diameter of 6 mm is recommended to avoid corrections for the effect of capillarity in water manometers.