Chlorophyll is the pigment in the chloroplast, so it won't be released.
Nitrogen is not entering the cell, so its lack of release can't be affected.
CO₂ is used in the Krebs cycle, the cycle to the right, and not by chloroplasts.
Oxygen is one of the by-products of photosynthesis, so it would be immediately affected if photosynthesis suddenly stopped.
Thus, the answer is Oxygen
