For a given substance the amount of light absorbed depends upon what four factors

For a given substance, the amount of light absorbed depend on these four factors:
1. The wavelength of light.
2. The extinction coefficient of the absorbing material.
3. The path lenght of the medium through which the light is travelling and
4. The concentration of the substance.

Most likely a is the answer - because  you generate   38 atp's but also consume an atp.   each atp gives approx. less than 4 kcal so probably a is the closest answer.