The correct option is a A.
Weathered debris refers to particles of rocks that are broken down. Frost wedging is the process by which rocks are broken down as a result of the freeze and thaw processes that occur continuosly inside them.
Most rocks have crevices in them which are filled with water, the water in these crevices freeze and melt as the temperature changes during the course of the day and night and this leads to breaking down of these rocks.