The wavelength of an electromagnetic waves is also known as the period. the number of waves that pass a given point in space. the negative of the frequency. the spatial distance between two successive troughs. all of these are true.

The sentence can be completed as follows:

The wavelength of an electromagnetic waves is the spatial distance between two successive troughs.

Note that the wavelength of a wave can also measured as the spatial distance between two successive crests of the wave. Also note that the second part of the sentence ("also known as the period") is not true, because period is another thing (in fact, the period is the time interval between two successive troughs).

it takes about 30 days for it to break down