The option that was NOT a contributing factor to the issue of slavery in the United States is:
The Missouri Compromise, which stated that there had to be one "free" state for every "slave" state.
While the Missouri Compromise did attempt to balance the number of free and slave states, it was more a political response to the ongoing issue of slavery rather than a direct contributing factor to the existence or continuation of slavery itself. The other options directly highlight factors that contributed to the entrenchment and expansion of slavery in the United States.