A schedule is not conflict-serializable. Is it serializable?
A schedule is considered serializable if it is equivalent to a serial one. The conflict graph that appears on the answer to item (a) is proof that the schedule is not conflict-serializable, and it is also not view-serializable, because there is still a cycle between $T_1$ and $T_3$, when considering just the history of the data they visualize. That means the schedule cannot be considered serializable at all.

Conflict-serializability is stronger than view-serializability. It is the criterion used for the implementation of most schedulers, and that is why the quiz emphasizes this point.