The correct choice is
d. radio waves.
For radio waves, wavelength is greater than 1 mm.
For infrared rays, wavelength lie in the range of 25 μm - 2.5 μm
For x- rays, wavelength lie in the range of 1 nm - 1 pm
For gamma rays, wavelength is less that 10⁻¹² m
From the above values of wavelengths, we see that radio waves have the longest wavelengths in the spectrum.
