The correct answer is A. 1 in 20.
A slope of 1 in 20 means that for every 20 feet of horizontal distance, the roof should rise 1 foot. This is the minimum slope required for effective drainage, as it ensures that water will flow off the roof and not pool.
Option B, 1 to 50, is not a valid slope. A slope of 1 to 50 would mean that for every 50 feet of horizontal distance, the roof should rise 1 foot. This is a very shallow slope and would not be effective for drainage.
Option C, 1 in 10, is a common slope for flat roofs. However, it is not the minimum slope required for effective drainage.
Option D, 1 in 5, is a very steep slope for a flat roof. It is not necessary for effective drainage and may actually make the roof more susceptible to leaks.
In conclusion, the minimum slope required for effective drainage of a flat roof is 1 in 20.