The statement, the disciples successfully cast out a demon from a father's demon possessed son after several hours' effort, is false.
The disciples were never able to fully cast out the demon from a father's demon possessed son until Jesus came and helped. Before the disciples attempt to drive out a demon but they fail to do so, so after which Jesus comes to help.
The nine disciples, however, were very sure that they could drive out the evil spirit from the father's son. It was because they had an experience of driving out evil spirits during their fieldwork training time. Still they could not do so.
Hence, option B is correct.
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