The answer is choice A. Roughly 72.8% of the variation in the finishing position can be explained by the runner's age.
All we do is move the decimal 2 spots to the right to convert to a percent. This is the same as multiplying by 100, so, 100*0.728 = 72.8%
There isn't any other work needed to be done because this is a useful property of the coefficient of determination.
Note: r^2 is the coefficient of determination while r itself is the correlation coefficient.