Answer:
Time period
Step-by-step explanation:
A disturbance that is created in a medium is called a wave. There are many characteristics of a wave i.e. period of wave, frequency, wavelength of a wave etc.
Time period of a wave is defined as the time required for a passage of one wavelength. It is denoted by T. Its relation with the frequency of wave is given as :

Frequency of a wave is equal to the number of oscillations or vibrations. Hence, the correct option is (a) " time period ".