Which phrase from the sentence gives it a formal tone?
we have before us an opportunity to make a change that will our children plan more completely for their retirement. (4 points)

The answer is C: we have before us.

The phrase “we have before us” means, colloquially, “to have”, and nothing else. However, the phrase, through time feeding meaning into language, has become a formal phrase used often in speeches of a persuasive nature. One historical example is the speech given by Churchill to the House of Commons of the Parliament of the United Kingdom on the 13th of May, 1940:

“I would say to the House as I said to those who have joined this government: I have nothing to offer but blood, toil, tears and sweat. We have before us an ordeal of the most grievous kind. We have before us many, many long months of struggle and of suffering.”