The question can be answered with a definitive scriptural reference from the book of Mark. The question is asking whether or not the disciples believed that Jesus Christ was the son of God. If the answer has to be limited to only the book of Mark, then you might be able to claim that the disciples didn't necessarily believe that Jesus was the son of God. They definitely believed that he was powerful, had done great miracles, and would save them from an oppressive government. Check Mark chapter 8 starting with verse 27. This section shows Jesus asking his disciples who the people believe that he is. Jesus then asks the disciples who they think Jesus is. Peter responds by saying that Jesus is the "Messiah." The Messiah is the promised deliverer of the Jews, but saying that Jesus is the Messiah doesn't automatically mean that they believed Jesus is the son of God.

You would need to check a different Gospel for a definitive answer. Look in Matthew chapter 16 starting with verse 13. Matthew narrates the exact same exchange between Jesus and his disciples, but Peter's answer differs in one key way. Peter declares that Jesus is the Son of God, and no disciple speaks against that idea. The common assent sends the message that they all agree with Jesus's divinity.

5 “But what about you?” he asked. “Who do you say I am?”

16 Simon Peter answered, “You are the Messiah, the Son of the living God.”