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 in the sewer. The self-cleansing velocity is a function of the sewer diameter, the slope of the sewer, and the type of solids in the sewer.
For a sewer with a diameter of 150 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.
If the sewer has a slope of less than 1 in 60, solids will deposit in the sewer and the sewer will become clogged. If the sewer has a slope of greater than 1 in 60, the sewer will be more expensive to construct and operate.
The other options are incorrect because they do not represent the minimum slope required for self-cleansing velocity.