Final answer:
The slave states' focus when they seceded from the Union was solely about slavery. Hence, the correct answer is option (B).
Step-by-step explanation:
The focus of the slave states when they seceded from the Union was B) Solely about slavery. While secessionists in the slave states did argue for states' rights, their main concern was the preservation of slavery. The declarations of secession clearly state that the protection of slavery, as an institution, was the primary motivation for leaving the Union.
This is supported by statements made by leaders of the Confederacy and the explicit mention of slavery in the secession documents.