The correct answer is: B. A photometer.
A photometer is an instrument used to measure the intensity of light. It can be used to compare the illuminating powers of two sources by measuring the amount of light reflected or transmitted by each source.
A spectrometer is an instrument used to measure the wavelength of light. It can be used to identify the different types of light present in a beam of light, but it cannot be used to measure the intensity of light.
A polarimeter is an instrument used to measure the polarization of light. It can be used to determine the orientation of the electric field of light, but it cannot be used to measure the intensity of light.
A fluxmeter is an instrument used to measure the radiant flux of a light source. Radiant flux is the total amount of energy emitted by a light source per unit time. It is a different quantity than illuminating power, which is the amount of light that is actually received by a surface.