Karst lakes are formed by…

A Collapsed caves
B Floods
C Glaciers

Some rocks are soluble over time by water, such as gypsum, limestone and dolomite, in a process called karstification. Caves formed of these rocks can collapse due to this process, and remaining depressions that fill with water are called karst lakes. Some of them are filled periodically, and some, like at Kirilai, are present year-round.

The correct answer is Collapsed caves.