Dissolution and precipitation of are the main processes that form limestone caves?
a. calcite
b. sandstone
c. clay
d. salt water

The answer is letter A. 
Dissolution and precipitation of calcite are the main processes that form limestone caves.

>Mineral precipitation from solution:Calcite dripstones form from calcite
saturated groundwater in cave.
> Limestones are produced from the mineral calcite (calcium carbonate) and sediment. The main source of limestone is the limy ooze formed in the ocean. They can be precipitated from ocean water or it can be formed from sea creatures that secrete lime such as algae and coral.