The minimum diameter of sewer to be adopted is 15 cm. This is because the sewer must be able to carry the maximum flow of sewage without overflowing. The maximum flow of sewage is determined by the number of people using the sewer and the amount of water they use. The larger the diameter of the sewer, the more water it can carry. Therefore, a 15 cm sewer is the minimum diameter that should be used to ensure that the sewer does not overflow.
Option A: 10 cm is too small a diameter for a sewer. A 10 cm sewer would not be able to carry the maximum flow of sewage and would likely overflow.
Option B: 12.5 cm is a slightly larger diameter than 10 cm, but it is still not large enough to carry the maximum flow of sewage. A 12.5 cm sewer would likely overflow if the maximum flow of sewage was reached.
Option C: 15 cm is the minimum diameter that should be used for a sewer. A 15 cm sewer can carry the maximum flow of sewage without overflowing.
Option D: 25 cm is a larger diameter than 15 cm, but it is not necessary. A 25 cm sewer would be able to carry more water than the maximum flow of sewage, but it would be more expensive to install.