Please help me Which meaning of den defines the word as it is used in the sentences?

Carlos was reading an interesting book about a group of pirates who lived long ago. He learned that after raiding other ships, they returned to their den to count their gold and avoid being captured.

A a secret place for having meetings

B a dirty, messy place where people live or gather

C a part of a club for boys

D a hiding place for thieves or robbers