When People Say That the United States Is a Nation

When people say that the United States is a nation of immigrants, what are they referring to?

A) The US Census documented many immigrants between 1850 and 1950 and then those numbers dropped off, thus our ancestors are immigrants.
B) The US Census has documented a steadily growing number of immigrants since the 1850 census, thus we have more and more immigrants with each passing decade.
C) The US Census has documented the entry of immigrants into the US with lower (4.4%) and higher (12.9%) rates depending on the decade. The US has had a steady influx of immigrants throughout its history.
D) The US Census did have very many immigrants between 1850 and 1980 and then suddenly numbers skyrocketed. Many of our current citizens are immigrants.