Answer: (iii) Directed Acyclic Graphs (DAGs)

Explanation:

• Topological sorting is a linear ordering of vertices such that for every directed edge $u \to v$, $u$ appears before $v$.
• Such ordering exists iff the graph has no directed cycle. If a cycle exists, no linear extension can satisfy all precedence constraints.
• Undirected graphs have no direction → concept not defined (except trivial cases). Trees (when oriented as DAG) can be topologically sorted, but not all trees as undirected structures.
• Weighted vs. unweighted is irrelevant; only acyclicity matters.

Algorithm: DFS post-order reversed or Kahn's BFS with indegree queue, both $O(V+E)$.

Correct choice: (iii)