Why is the traversal of a graph different from a tree?

a. There can be a loop in a graph so we must maintain a visited flag for every vertex
b. DFS of a graph uses stack, but in-order traversal of a tree is recursive
c. BFS of a graph uses queue, but a time efficient BFS of a tree is recursive
d. All of the above

a. There can be a loop in a graph so we must maintain a visited flag for every vertex
To avoid running into a never-ending loop, we use a Boolean array of visited vertices.