Answer:
Chlorophyll
Chloroplast
Step-by-step explanation:
Chlorophyll is a pigment found in the chloroplast organelle of certain organisms including plants, algae etc. Chlorophyll is the reason plants and certain algae are able to perform photosynthesis because chlorophyll pigment helps them capture light energy from the sun.
According to this question, a xerophytic plant called CACTUS, is being described to possess a chemical in one of its organelle that makes it seem green in appearance. That chemical is CHLOROPHYLL, with chemical formula, C₅₅H₇₂O₅N₄Mg. It is found in the CHLOROPLAST organelle as stated earlier and makes the stem of cactus and other plants green because it reflects green light and absorbs other colour of light. This makes organisms that possess chlorophyll appear GREEN.