Answer
Constructive interference is required.
Work Step by Step
The electron needs to form a standing wave around the circumference, and this requires a whole number of wavelengths so that the wave closes and matches up with itself. With 2.5 de Broglie wavelengths around one circumference, there would be destructive interference, and there would be no standing wave and no electron.
This is discussed on pages 608-609, and shown in Figure 32.10.
