Which of the following, is the longest wave?

(a) X-rays

(b) y-rays

(c) Microwaves

(d) Radio waves

(d)

        Rays                  Wavelength [Range in m]

        X-rays               1 × 10^-11 to 3 × 10^-8

        Y-rays               6 × 10^-14 to 1 × 10^-11

        Microwaves        10^-3 to 0.3

        Radio waves       10 to 10⁴