Answer : 4. "प्रति इकाई लम्बाई में तरंग -दैर्घ्य कि संख्या को"
Explanation :

A wave number, or wavenumber, refers to the spatial frequency of a wave at a specific unit distance. Wave number is measured either in cycles per unit distance or in radians per unit distance, depending on the context. A wave number is related to but distinct from frequency, which is temporal in nature.