What was the largest and most constant issue facing the united states during the era of manifest destiny?

It was the quest for westward expansion. The Manifest Destiny called for the US to fulfill the divine mission to expand and dominate the whole continent. After the independence, the US moved westward step by step: the Louisiana Purchase, Texas, the Mexican-American War. This last event allowed the US to increase its territory, incorporate California and reach the shores of the Pacific Ocean.

Explanation:

option d is correct. corroborated sources of historical evidence   is necessary to validate a claim about an event in history.

historians tend to use written sources, but audio and visual materials as well as artifacts are important objects that supply information to modern historians.

historical sources are usually divided into two categories: primary and secondary sources. depending on the historian's intent, some sources shift their designation. determining what type of sources toimplement, and the level of credibility and reliability of those sources, is a significant step in critical thinking for the historian.

explanation: