The correct answers are caves and sinkholes.
A topography produced due to the dissolution of soluble rocks like dolomite, limestone, and gypsum is known as Karst. It is featured by the underground drainage mechanisms with caves and sinkholes. It has been also recognized for more weathering-resistant rocks like quartzite given the right conditions.
The examination of karst topography is regarded of prime importance in petroleum geology as many of 50 percent of the globe's hydrocarbon reserves are accommodated in karst organizations.