The correct answer is option A, that is, acid rain.
Karst refers to a creation of topography by the dissolution of soluble rocks, like dolomite, limestone, and gypsum. It is featured by underground drainage systems with caves and sinkholes. The development of karst takes place whenever acidic water begins to dissociate the surface of bedrock close to its cracks, or bedding planes.
The carbonic acid, which makes karstic features is produced as rain goes through the atmosphere picking up carbon dioxide that dissolves in the water. Once the rain goes inside the ground, it may move through the soil, which can offer much more CO2 to produce a weak carbonic acid solution that dissolves calcium carbonate.
Other mechanisms, like acid rain, can also result in karst formation. The oxidation of sulfides resulting in the production of sulfuric acid can also be one of the corrosion factors in karst creation.