What is the sum of the interior angles of a regular hexagon?

The interior angles of an $n$-sided polygon sum to $(n-2)(180^\circ)$. For a hexagon, $n=6$, so the sum is $4(180^\circ)=720^\circ$, choice J. You can see this by drawing diagonals from one vertex, which splits a hexagon into $4$ triangles. Choice F ($540^\circ$) uses $n=5$, the pentagon. Choice H ($900^\circ$) uses $n=7$, the heptagon. Choice G ($1080^\circ$) uses $n=8$, the octagon. Each wrong choice corresponds to a polygon with the wrong number of sides, so count the sides carefully. Each interior angle of a regular hexagon is $120^\circ$.