The correct answer is D: 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.
The Compromise of 1850 was designed to address the issues arising from the acquisition of new territories after the Mexican-American War. While it did admit California as a free state, it also included provisions that upheld stricter measures for the recapture of runaway slaves, which was a significant concern for Southern states. This illustrates that Southern states were focused not only on the immediate status of slavery in individual states but also on protecting their interests through federal legislation.