the ideal amount of nitrogen fertilizer

To efficiently provide the ideal amount of nitrogen fertilizer, some farmers make use of a “chlorophyll meter” that measures the amount of green light reflected off a leaf.
Why does this provide an indication of whether the plant may need more or less nitrogen?

A. Nitrogen ions form the electrochemical gradient across the thylakoid membrane that drives ATP synthesis.

B. Beta-carotene is composed largely of nitrogen atoms and has a similar absorption to chlorophyll.

C. Nitrogen gas is green, and photosynthesis uses N2 to run the Calvin cycle.

D. Each chlorophyll molecule contains four nitrogen atoms that surround the Mg2+ ion and these account for most of the nitrogen in a leaf.