Answer
The airplane's shadow falls in the center of the circle.
Work Step by Step
As described on page 530, the rainbow you see is from droplets that lie within a 3-D cone, with your eye at the tip.
When people on the airplane look down at a cloud of droplets, in a direction opposite from the sun, they may see a complete rainbow. As illustrated in Figure 28.33, a straight line can be passed through the sun, the observer's eye, and the center of the circular rainbow (i.e., this line is the axial line of symmetry for the 3-D cone).
This means that the plane's shadow falls squarely in the center of the rainbow (or perhaps we should say it falls circularly in the center, lol?).
