Quiz: Detection Logic

You're checking bipartiteness with BFS. You color node $u$ as $0$. When visiting neighbor $v$, you find $v$ is already colored $0$. What does this mean?

1. A.The graph is bipartite
2. B.You should skip this edge
3. C.You need to recolor $v$
4. D.You found an odd cycle. The graph is NOT bipartite