Answer:
Option A. The U.S. Constitution.
Explanation:
The reason is that the US constitution provides the basic framework for the laws to be formulated and passed both in the US National Assembly and the Senate. The laws cannot be formulated if the US constitution doesn't allows the passing of the law which means that the constitution must be amended first. When the slavery was to be eliminated in 1865, the first problem was that passing the elimination of slavery law was inconflict with the US constitution. So the constitution was amended before passing the law to eliminate the conflict.
In the nutshell, every law originates because of different reasons like Technological reasons, Social reasons, Cultural reason, etc) but the basis remains the same which is US constitution which is the basic framework for passing the law.