Final answer:
South Carolina was the only southern state to pass a unanimous vote for secession, with a convention vote of 169-0 on December 20, 1860.
Step-by-step explanation:
The only southern state to pass a unanimous vote for secession was South Carolina. On December 20, 1860, South Carolina's secession convention voted unanimously 169-0 in favor of dissolving their Union with the United States. The election of Abraham Lincoln and the perceived threats towards the institution of slavery incited quick actions by the southern states.
Following South Carolina, other states such as Mississippi, Florida, Alabama, Georgia, Louisiana, and Texas also seceded, but none of their votes were unanimous, indicative of the divisions within those states regarding the secession decision.