What act set the tone for the westward expansion of the United States of America by establishing that new states would be formed and admitted to the United States, instead of allowing existing states

What act set the tone for the westward expansion of the United States of America by establishing that new states would be formed and admitted to the United States, instead of allowing existing states to expand their territory? A) the Compromise of 1850 B) the Northwest Ordinance C) the Missouri Compromise D) the Kansas-Nebraska Ordinance