The correct answer is (d) α = 20 log(A0/A)/X
To explain I would say: Ultrasonic attenuation coefficient (α) is calculated from the following equation-
α = 20 log(A0/A)/X
Where, A0 and A are the ultrasound amplitudes before and after travelling through the material, and X is the travel distance. The measurement of attenuation coefficient is mainly used to find qualitative and quantitative information on the internal structure of products or castings.