The obvious choices are to make an anchor--the 21 point, or to come in and hit red off the 9 point. Years ago Joe Sylvester taught me a great rule of thumb: "offense/offense; defense/defense." What he means is that if you find yourself on the offensive, tend to choose an offensive play if you can, and if you find yourself on the defense, make a defensive play. Blue is clearly on the defense here....well behind in the race, fewer points in his inner board, and more checkers back.
Making an anchor solidifies Blue's defensive position and prevents the possibility of being blitzed. Generally, you don't want to get into a hitting exchange when your opponent has more inner board points than you do, and making your opponent's 4 point, a point in advance of the made 2 point, makes that two point less valuable to Red.