Slaves were bound to their master's lands, until one's debt was paid to his master. Barring being a captive, being punished for committing a crime, or failing to pay an outstanding gambling debt, slavery was an institution one could enter into freely.
Although you unhelpfully didn't supply the following choices, the answer is
<u>the opportunity for former slaves to leave the U.S.</u>
It's means that there are other laws even though they are not written there
