Answer:
C. 0.2 Hertz
Explanation:
The frequency of a spring is equal to the reciprocal of the period:

where
f is the frequency
T is the period
For the spring in this problem,
T = 5 s
therefore, the frequency is

The correct answer is "wavelength".
In fact, this is exactly the definition of wavelength: in a wave, the wavelength is defined as the distance between two succesive crests or two successive troughs, therefore the correct answer is
D) wavelength.