The correct answer is B. 4.
A well point is a small, shallow well that is used to remove groundwater. It is typically made of perforated pipe that is surrounded by a filter screen. The well point is installed in the ground and a pump is used to remove the water.
The number of well points that need to be installed depends on the depth of the excavation, the type of soil, and the amount of groundwater that needs to be removed. In general, well points should be installed at intervals of 5 to 10 meters.
For an excavation that is 20 meters deep, four well points should be installed at depths of 5, 10, 15, and 20 meters. This will ensure that the groundwater is removed evenly from the excavation.
Option A is incorrect because it would require too many well points. Option C is incorrect because it would not provide enough coverage. Option D is incorrect because it would not provide enough depth coverage.