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

Here's why:

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

* Acidic Reaction: When limestone comes into contact with slightly acidic groundwater, a chemical reaction occurs:

* CaCO3 (limestone) + H2O (water) + CO2 (carbon dioxide) → Ca(HCO3)2 (calcium bicarbonate)

* Solubility: Calcium bicarbonate is soluble in water, meaning it dissolves, leading to the gradual erosion of the limestone.

This process is known as karst topography, and it's responsible for the formation of caves, sinkholes, and other unique geological features.