The correct choice is
A. wavelength.
The frequency of electromagnetic wave is given as
[tex]f=\frac{c}{\lambda }[/tex]
where "c" stands for speed of light whose value is well known as 3 x 10⁸ m/s and λ stands for wavelength of wave.
hence to determine the frequency, we need wavelength of wave. hence the correct choice is wavelength.
