The disciples could not cast out the dumb spirit because
It was stubborn
They were not familiar with that particular spirit
They had not been fasting
It could only be driven out by prayer
Correct answer is D
This incidence can be found in Mark 9:14-29. In this passage, a man brings his son possessed by a mute spirit to the disciples, asking them to cast out the spirit. The disciples attempt to do so but are unsuccessful. When Jesus arrives, he casts out the spirit and later explains to his disciples that such spirits can only be driven out by prayer.