Answer: B. keep the balance of power in the Senate between the slave states and free states.
Further detail:
The Missouri Compromise (1820) admitted Missouri into the Union as a slave state with Maine being added at the same time to keep the balance of slave and free states equal. It also prohibited any future slave states north of the latitude line 36 1/2 degrees north of the equator in territories of the Louisiana Purchase, with the exception of Missouri (north of that line) being admitted as a slave state.
Engel v. Vitale, 370 U.S. 421 (1962), was a landmark United States Supreme Court case that ruled it is unconstitutional for state officials to compose an official school prayer and encourage its recitation in public schools.