The correct response is: The southern states accepted the compromise because, despite allowing California to become a free state, it strengthened federal fugitive slave laws, indicating that they were most concerned about the status of slavery at the federal level.
This choice accurately reflects the concerns of southern states during the Compromise of 1850, as they were primarily focused on ensuring the security of slavery and its expansion into new territories while also safeguarding their interests at the federal level. The compromise allowed California to enter as a free state, but the reinforcement of fugitive slave laws was seen as a way to protect southern slavery.