The correct answer is option 3, "Man - Homo sapiens." This option refers to human beings, who are indeed the most populous mammal species on Earth.

Option 1, "Elephant," is not the commonest mammal. While elephants are large and well-known animals, their population is smaller compared to humans.

Option 2, "Lion," is also not the commonest mammal. Lions are social big cats that are found in certain regions of Africa and Asia, but their numbers are much lower compared to humans.

Option 4, "Panther," is a term used to describe large cats with black fur, such as leopards or jaguars. However, just like lions and elephants, their populations are significantly smaller than that of humans.

Therefore, the correct answer is option 3, as humans are the most numerous mammal species on Earth.