A. the Compromise of 1850 was illegal because it was illegal for Congress to deprive a slave owner of property (slaves) without the due process of law.
Under the Compromise of 1850, western states were allowed to vote whether or not territories would allow slavery. The compromise also created a dividing line for existing states between free and slave states.
The Dred Scott case was decided in 1857 after Scott's master had moved him into free states. He argued that by being moved into free states, declared after the Compromise of 1850, changed his status. The Court declared slaves were not people but were property. Due to their status as property, the property could not be taken away just because the master moved.