Refer to figure.
Radio waves are electromagnetic waves that travel outwards from a transmitter at the speed of light. They are transverse waves, meaning that the electric and magnetic fields involved travel from side to side (or up and down) whilst the wave goes "forwards".

There are a few components of these waves that you are required to know. Two of the most important are wavelength and frequency.

Wavelength and frequency are linked by the formula:

c = f x λ

Where;
c = the speed of light (m/s) = 3x10⁸ m/s
f = frequency (Hz)
λ (Lambda) = wavelength (m)

An Automatic Direction Finder (ADF) is the airborne receiver for NDB signals, and tells the pilot from which direction the NDB signal came, allowing for navigation. Receivers are made with a particular range of wavelengths in mind, to have the highest accuracy and efficiency.

NDBs operate in the frequency range of 190 kHz to 1750 kHz. When inputted into the formula above, (rearranged to λ = c/f), this gives a wavelength range of 171 m to 1578 m.

For this type of question, we need to use terms for how long that is (in orders of magnitude). So 1-10 m is metric (for metre), 10-100 m is decametric (deca meaning 10), 100-1000 is hectometric, 1000-10 000 is kilometric, etc. NDBs/ADFs operate in the hectometric and kilometric range.

The terms go both higher and lower than metric, see the above figure.