Assertion: Patriliny had existed prior to the composition of the Mahabharata epic.
Reason: The central story of the Mahabharata reinforced the idea that Patriliny was valuable.

Both the Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

While patriliny had existed prior to the composition of the epic, the central story of the Mahabharata reinforced the idea that it was valuable.
Patriliny means tracing descent from father to son, grandson and so on. Matriliny is the term used when descent is traced through the mother.