Proper frequency of a wave Can I get a brief explanation as

Proper frequency of a wave?
Can I get a brief explanation as well

If one wants to determine the proper frequency of a wave, which of the following statements is true? The proper frequency must be measured in the same frame as the proper length is measured. The proper frequency must be measured in the same frame as the proper time is measured. Choices (a) and (b) are both correct. None of the above answers are correct.

Solution

Frequency which is measured by the observer in the rest frame is called proper frequency..

so the choices both (a) and ( b) are correct