As a result of the missouri compromise, a. maine entered the union as a slave state and missouri entered as a free state. b. maine entered the union as a free state and missouri entered as a slave state. c. both maine and missouri entered the union as slave states. d. both maine and missouri entered the union as free states.

B the MO compromise made MO a slave state and Maine a free state

B. Maine entered the Union as a free state and Missouri entered the Union as a slave state.

Further details:

The Missouri Compromise (1820) admitted Missouri into the Union as a slave state with Maine being added as a free state 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.