Question

Assertion: Cro-Magnon man is regarded as the closest ancestor of today's humans.
Reason: They had the largest cranial capacity among ancestors of humans.

Answer 1

Homo sapiens fossilis commonly known as Cro-Magnon man emerged around 34000 years back. They were omnivorous, with perfectly orthognathous face, elevated nose, broad and arched forehead, moderate brow ridges, strong jaws with man-like dentition, and a well-developed chin. They had a cranial capacity of 1650cc (largest of all known human ancestors including modern man). They were the first to domesticate animals. Thus, they are regarded as the most recent ancestor of today's man. Cranial capacity alone does not determine the closeness of ancestry but all the above features combined make it possible to trace their close relationship to modern humans.

So both the assertion and reason are true but the reason is not a correct explanation of the assertion.