The correct answer is B. To increase it.
The self-cleansing velocity of a sewer is the minimum velocity required to keep the sewer clean and free of deposits. The self-cleansing velocity is affected by a number of factors, including the diameter of the sewer, the slope of the sewer, the roughness of the sewer walls, and the type of wastewater being conveyed.
Increasing the diameter of a sewer will increase the self-cleansing velocity. This is because the larger diameter will allow for a higher flow rate, which will in turn create a higher velocity. The higher velocity will be more effective in scouring the sewer walls and keeping the sewer clean.
The other options are incorrect. Option A is incorrect because increasing the diameter of a sewer will increase the self-cleansing velocity. Option C is incorrect because the self-cleansing velocity is not fluctuating. Option D is incorrect because the self-cleansing velocity is affected by the diameter of the sewer.