Frost wedging weathers rock because

water expands when it freezes
oxygen expands when it is heated
water contracts when it freezes
plant roots create external pressure

water expands when it freezes

Weathering is a phenomena in which a rock is broken down into small parts and sediments, due to the action of wind, water and chemical agents. Frost wedging is a physical weathering process. It occurs in extremely cold regions. It involves rapid freezing and thawing of water present inside the rocks, because of change in the weather conditions. When water inside the rocks freezes, it changes it's form from liquid to solid. In the solid form, water present in the ice expands, which results in the internal breakage of the rock. It causes the partial splitting of rock, acquiring the shape of a wedge. Therefore, the phenomena called as frost wedging.

the question isn't complete here so i can't right an answer '_'