The disciples could not cast out the dumb spirit because

This question is asking why the disciples were unable to cast out a dumb spirit. The options given are: A) it was stubborn, B) they were not familiar with that particular spirit, C) they had not been fasting (which is the correct answer), and D) it could only be driven out by prayer.

According to the Bible, in Mark 9:28-29, the disciples asked Jesus why they were unable to cast out the dumb spirit from a boy. Jesus replied that it was because of their lack of faith and prayer. In Matthew 17:21, Jesus specifically mentioned that some kinds of spirits can only be cast out by prayer and fasting.

Fasting is a spiritual discipline that involves abstaining from food or other pleasures for a period of time in order to focus on prayer, worship, and seeking God's will. Through fasting, Christians seek to deepen their relationship with God, and gain spiritual strength and insight.

In this case, the disciples had not been fasting, and therefore lacked the spiritual strength and insight needed to cast out the dumb spirit. This shows the importance of spiritual disciplines like fasting and prayer in the life of a Christian.

In conclusion, the correct answer to this question is option C: they had not been fasting. This emphasizes the importance of spiritual disciplines like fasting and prayer in the life of a Christian.