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.
