The correct answer is: C. Both (A) and (B)
Negative skin friction is a downward drag acting on a pile due to downward movement of the surrounding compressible soil relative to the pile. It develops due to lowering of ground water.
When the ground water level is lowered, the soil above the water table becomes unsaturated. This causes the soil to shrink and consolidate, which in turn causes the soil to move downward relative to the pile. This downward movement of the soil creates a negative skin friction force on the pile.
Negative skin friction can be a significant problem for piles that are used to support structures. If the negative skin friction force is large enough, it can cause the pile to uplift or even fail.
There are a number of ways to mitigate the effects of negative skin friction. One way is to use piles that are designed to resist uplift forces. Another way is to install the piles below the ground water table.