The correct answer is A. 1 in 60.
The self-cleansing velocity of a sewer is the minimum velocity required to prevent the deposition of solids on the sewer walls. The self-cleansing velocity is a function of the sewer diameter, the roughness of the sewer walls, and the type of solids in the wastewater.
For a sewer with a diameter of 225 mm, the self-cleansing velocity is 1 in 60. This means that the sewer must have a slope of 1 inch per 60 inches, or 0.0167 degrees.
The other options are incorrect because they do not provide a slope that is sufficient to generate self-cleansing velocity. Option B, 1 in 100, would result in a slope of 0.0167 degrees, which is not sufficient to generate self-cleansing velocity. Option C, 1 in 120, would result in a slope of 0.0133 degrees, which is also not sufficient to generate self-cleansing velocity. Option D, None of these, is incorrect because it does not provide a correct answer.