Correct choice is (b) Power gain
To explain I would say: The ratio of power radiated in a particular direction to the actual power input to antenna is called Power gain. (G_p=frac{P_{d( heta,emptyset)}}{P_{in}}). Directive gain is the ratio of power radiated in desired direction to the average power radiated from the antenna. Partial directivity is the part of radiation intensity in a particular polarization to radiation intensity in all directions.
