Answer is D.
This problem is finding the largest maximal independent set or maximum independent set.
Independent Set : Given an undirected Graph G = (V,E) an independent set is a subset of nodes U ⊆ V , such that no two nodes in U are adjacent.
Maximum Independent set: An Independent set of maximum cardinality is known as maximum independent set.
Maximal Independent set: An independent set that is not the subset of another independent set is called maximal.