The type of rock most commonly dissolved by slightly acidic groundwater is limestone.

Here's why:

* Composition: Limestone is primarily composed of calcium carbonate (CaCO₃).

* Chemical Reaction: Slightly acidic groundwater (containing dissolved carbon dioxide, forming carbonic acid) reacts with calcium carbonate in a process called carbonation. This reaction forms calcium bicarbonate (Ca(HCO₃)₂), which is soluble in water.

The chemical equation for this reaction is:

CaCO₃ (s) + H₂O (l) + CO₂ (aq) → Ca(HCO₃)₂ (aq)

This dissolving process creates karst topography, characterized by caves, sinkholes, and underground drainage systems.

While other rocks like marble and dolomite (which also contain calcium carbonate) can be dissolved by acidic groundwater, limestone is the most prevalent and susceptible rock type.