The Centre has reconstituted an (A)/advisory committee to chalk out a plan…

Correction: replace “mythics” with “mythical.”

Explanation:

• “Mythics” is a noun (plural) meaning myths; it does not function as an adjective to modify “Sarasvati River.”

• Use an adjective instead: “mythical” (more common) or “mythic” are correct adjectival forms. “Mythical Sarasvati River” is the most natural phrasing.

Corrected sentence:

The Centre has reconstituted an advisory committee to chalk out a plan for studying the mythical Sarasvati River for the next two years, after the earlier panel's term ended in 2019.

Note: The other parts of the sentence — “reconstituted,” the expression “chalk out a plan,” and the use of “earlier” — are all correct.