Final answer:
Lake Mungo in Australia is the ancient lake associated with the evidence of long-term human cohabitation and the discovery of stone tools alongside megafauna.
Step-by-step explanation:
The ancient ephemeral lake that offers potential evidence of approximately 10,000 years of cohabitation, featuring the discovery of stone tools in association with megafauna, is Lake Mungo (A). Lake Mungo, located in Australia, is known for its archaeological significance due to the discovery of human remains and evidence suggesting an extensive history of human occupation alongside a variety of now-extinct Australian megafauna.
Archaeologists have found various stone tools and fossils in the area that provide insights into the early human technologies used during the Paleolithic period. These findings highlight not only human history but also the climatic conditions and the environment in which these ancient populations lived.