Sinkholes and karst topography are a clear indication that

Presence of carbonate rocks such as limestone and flow of groundwater

Explanation:

Sinkholes and karst topography are two of the distinct geological feature that is comprised of carbonate rocks such as limestone. Sinkholes are formed due to the dissolution of rocks below the earth's surface. These rocks when it comes in contact with the water, dissolve the minerals and leads to the formation of small underground caves. These form the path of groundwater flow.

Thus, the presence of sinkholes and karst topography clearly indicates the presence of groundwater and where the rocks are carbonate in nature.