The one parameter that determines the brightness of a light source sensed by an eye is
(A) Energy of light entering the eye per second
(B) Wavelength of the light
(C) Total radiant flux entering the eye
(D) Total luminous flux entering the eye

Hint : To answer this question we have to look into the basic definitions of each of the quantities mentioned in the options. Then we have to investigate which quantity is related to the brightness of light sensed by an eye.

Complete step by step answer
We know that the energy of light radiating per second on a surface is known as the power. So the energy of light entering the eye per second is nothing but the power of the light incident on the eye. But the energy of light which is entering the eye can be of any wavelength. But according to the question, the light must be sensed by the eye. So the wavelength must correspond to the visible region of the electromagnetic spectrum.
Therefore, option A is incorrect.
The wavelength of the light is a characteristic for a given light. It does not depend upon the brightness of the light. So the brightness of the light cannot be determined by its wavelength.
Therefore the option B is also incorrect.
The radiant flux denotes the total energy per unit time that is emitted or received by a surface. So basically, the radiant flux measures the total power which is carried by electromagnetic radiation. As already stated above, the electromagnetic radiation can belong to any region of the EM spectrum, and not necessarily to the visible region.
So the option C is also incorrect.
The luminous flux measures the variation of the sensitivity of the human eye to the different wavelengths of the light. For measuring the sensitivity of the human eye, the light must belong to the visible region. The luminous power flux is basically the light power perceived by the eye. We know that the power of a light is a measure of its brightness. Therefore the total luminous flux entering the eye determines the brightness of a light source which is sensed by an eye.
Hence, the correct answer is option D.

Note
The SI unit of the luminous flux is lumen. It is an important criterion in the specifications of lamps under the IEC standards and is mentioned on the packaging of the bulbs.