Right answer is (a) N = 10 log I1/I2
For explanation: If I1 and I2 are two intensities in watts per square centimetre, then the number of decibels with which they are related can be expressed as N = 10 log I1/I2. Attenuation is commonly expressed in negative dB numbers whereas amplification is given in positive dB numbers. Use of decibels as units for comparison of intensities help to avoid all mathematical calculations except algebraic addition or subtraction of small numbers. The transmission efficiency of any medium like air, a hearing aid or an amplifier is usually expressed in dB as a gain when the output is greater and as a loss, if less.