The correct answer is A. 1 in 60.
The minimum gradient for house drainage is 1 in 60. This means that for every 60 feet of horizontal run, the pipe should drop 1 foot vertically. This is necessary to ensure that the wastewater flows freely and does not back up.
Option B, 1 in 80, is not a common gradient for house drainage. It is possible to use this gradient, but it is not recommended. Option C, 1 in 10, is too steep and can cause the wastewater to flow too quickly. This can lead to erosion and damage to the pipe. Option D, 1 in 400, is too shallow and will not allow the wastewater to flow freely.
It is important to note that the minimum gradient for house drainage may vary depending on the local building codes. It is always best to consult with a licensed plumber to determine the correct gradient for your home.