The type of rock most commonly worn away by weak acid in water to create a cave is limestone.

Here's why:

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

* Reaction with acid: Weak acids, like carbonic acid (formed when carbon dioxide dissolves in water), react with calcium carbonate. This reaction produces calcium bicarbonate (Ca(HCO3)2), which is soluble in water.

* Dissolution: The soluble calcium bicarbonate is dissolved and carried away by the water, leaving behind empty spaces in the rock. Over time, these spaces grow larger, forming caves.

This process is called karst topography and creates a variety of unique landforms, including caves, sinkholes, and underground rivers.