Explanation:

- Let’s break it down:

- Option 1: Chlorophyll — This is the green pigment in plants. Chlorophyll helps them trap sunlight and carry out photosynthesis. It's literally the reason leaves look green.

- Option 2: Urea — This is a nitrogen-rich fertilizer. It helps plants grow, but it’s got nothing to do with their color.

- Option 3: Carbon dioxide — Plants use carbon dioxide from the air during photosynthesis, but it doesn’t give them color.

- Option 4: Oxygen — Plants release oxygen as a byproduct of photosynthesis. Again, this has nothing to do with color.

So, chlorophyll’s the answer. That green you see? It’s all thanks to it.