Many americans in the 19th century believed in “manifest destiny.” which of the following is true description of this concept?
a. mexico was destined to be an enemy of the united states unless they were dealt with early, before they were free of spanish influence.
b. many believed that it was god’s will and an obvious truth that the united states was destined to expand westward and grow.
c. the united states needed an explanation or excuse for its expansionist goals. this was simply such an excuse; no one really believed it.
d. protestants in the united states felt that it was god’s will that they expand at the expense of catholic governments such as in mexico.

This question is tough to answer, since perceptions of Manifest Destiny changed radically across the 19th century.

But many American citizens, politicians, and thinkers genuinely believed in the tenets of Manifest Destiny, so it's not fair to say that these Americans were simply manufacturing a false excuse for westward expansion. So we can exclude C.

It's also true that many other Americans (especially Southern Democrats) used the idea of Manifest Destiny to justify invading Mexico in the 1840s. Bu these Southerners were more interested in adding new slaveholding states to the Union than they were with fending off a potential enemy in Mexico (which was a vastly weaker military power).

And while much of America throughout the 19th century was indeed Protestant, and that most of the residents of Mexican territories were Catholic, Manifest Destiny was less interested in dismantling Catholic influence than it was in advancing its own expansionist, Protestant interests. 

You'll want to double-check with your textbook to be sure about the context of this question, but the best answer from this angle seems to be B, since those Americans who did believe in Manifest Destiny certainly believed that westward advancement was not only obvious but sanctioned by God.