The correct answer to this open question is the following.
Although there are no options attached we can say the following.
What does O'Sullivan mean when he states, "For this blessed mission to the nations of the world, which are shut out from the life-giving light of truth, has America been chosen…"?
What O'Sullivan meant with that expression was that the United States deserved a bright future and a great destiny. He thought that America has been chosen to be the greatest nation with God's blessing and he coined the term "Manifest Destiny" to refer to the faculty of the United States to expand and grow, not taking into consideration other territories or nations affairs.
John O'Sullivan was a Journalist who wrote articles in the "Democratic Review" in 1845, that favored the annexation of Texas to the Union. In his columns, he argued in favor of the US to expand and fulfill its manifest destiny to Be great.