Final answer:
The most important discovery made by humans approximately 2.5 million years ago was the control of fire, which was a significant advancement for human evolution and survival.
Step-by-step explanation:
The most important discovery that humans made about 2.5 million years ago was B. Control of fire. The invention of agriculture happened much later, around 10,000 BCE. This crucial discovery of controlling fire provided our ancestors with heat, protection, and the ability to cook food, which greatly impacted their survival and development. While the exact time of this discovery remains debated, it is clear that by at least 125,000 years ago, modern humans had developed the ability to start and control fires. This was a significant step in human evolution, as it led to the development of creative and organized societies and marked a cornerstone for human advancement.