Which is the first delegate selection event in every presidential election since 1972?

The correct answer is Iowa caucuses.

The Iowa Caucuses are quadrennial electoral events in which members of both the Democratic and Republican parties in the U.S. state of Iowa meet to select delegates who will vote for their party's nominee in the United States presidential election at the party convention. Since 1972, the Iowa caucuses have had a 43% success rate at predicting which Democrat, and a 50% success rate at predicting which Republican will go on to win the nomination of their political party for president at that party's national convention.