The correct option is Permeable and soluble.
Explanation
Karst topography refers to a distinctive landscape formed primarily by the chemical weathering (dissolution) of soluble sedimentary rocks. This process is driven by groundwater acting as a geomorphic agent.
Analysis of Requirements:
- Solubility: The bedrock must be chemically soluble in water. The most common rocks for Karst development are limestone and dolomite, which are rich in calcium carbonate. Rainwater absorbs carbon dioxide from the atmosphere and soil to form weak carbonic acid, which dissolves these rocks.
- Permeability: The rock must be permeable to allow water to penetrate and circulate. In the context of Karst, this permeability is typically secondary, resulting from joints, fractures, and bedding planes. These openings allow water to infiltrate deep into the rock mass, enlarging cracks into caves and underground channels.
- Incorrect Options:
- Impermeable and insoluble: If the rock is impermeable, water flows over the surface (surface runoff) rather than infiltrating, preventing the formation of underground drainage. Insoluble rocks do not undergo the necessary chemical dissolution.
- Hard and resistant: While limestone can be hard, "resistant" usually implies resistance to weathering. Karst requires rocks that are specifically non-resistant to chemical weathering.
- Volcanic and igneous: These rocks are typically composed of silicates, which are not readily soluble in weak natural acids and do not form Karst landscapes.
Key Takeaway:
The essential prerequisites for Karst topography are soluble bedrock (like limestone or dolomite) and sufficient permeability (via joints and cracks) to facilitate the circulation of groundwater.