The correct choice is (a) 20 log10 (V1/V2)
For explanation I would say: If V1 is the voltage at port 1 and V2 is the voltage at port 2, then the attenuation in dB is Attenuation in dB = 20 log10 (V1/V2) where V1 is the voltage at port 1 and V2 is the voltage at port 2.