Final answer:
Option A). The largest gravitationally bound structure in the universe is a cluster, specifically the Virgo Supercluster which includes our Milky Way galaxy within the Local Group.
Step-by-step explanation:
A supercluster is the largest known gravitationally bound structure in the universe. Superclusters are massive structures that can span several hundred million light-years, formed when galaxy clusters are grouped together. The Local Group, which is a subgroup of the Virgo Supercluster, includes our own Milky Way galaxy. This supercluster covers an area of roughly 110 million light-years in diameter. Large-scale filamentary structures that are found throughout the universe are formed when clusters of galaxies, such as the Local Group, combine with other clusters to form these massive supercluster structures.