A) The luminous efficiency of a monochromatic source may be less than that of the white light if the former emits wavelength far away from 555 nm.

B) Yes, it is true that our eyes mostly respond to colours close to the wavelength of 555 nm and detect them bright. So, luminous efficiency is unity (highest).

C) It is true because white light distributes its energy amongst certain colours that our eyes cannot detect as brightly as they detect a 555 nm light.

D) It is not necessarily true. If the monochromatic light radiates in a wavelength that is far away from 555 nm, our eyes will not perceive it as bright. So, it will have lesser illuminating power.