Fog, the other three options would be too high in the atmosphere to cause visibility problems.
The idea that O'Sullivan describes in this quotation is A) manifest destiny. Manifest Destiny was an idea that O'Sullivan was responsible for popularizing in the 1800s. This political theory or ideology sought for the United States to expand its borders to the Pacific Coast based upon a god given right to expansion which he called Manifest Destiny.
What I can infer from what Lewis and Clark saw when moving west from the Mississippi river is that the landscape along the river changed drasticallly. The forest receded, replaced first by the high prairie and then the shorter grass of the high plains. The temperatures became colder and as they continued their expedition by the river they realized that there was no water route to the pacific ocean, just more mountains.
