Answer:
The correct answer is A. In the period before the Civil War, the principle of popular sovereignty meant that whether or not a territory allowed slavery would be left up to territorial legislatures.
Step-by-step explanation:
The principle of popular sovereignty implied that, since the Compromise of 1850, the decision of whether or not to accept slavery in their territories was available to the states. Each state, in particular through its respective legislatures, could vote on the issue of slavery, so that it was popular sovereignty, that is, the will of the people through their representatives that would determine the issue.