Tomato and bell pepper follow the ordinary C3 route of carbon fixation, and this question asks what actually happens to them when they are grown with extra carbon dioxide around their leaves.
- C4 pathway is the limiting factor: Not applicable, since tomato and bell pepper never use a C4 pathway at all, they are C3 crops.
- Increased photosynthesis rate at higher CO2: This fits the science. In C3 plants the RuBisCO enzyme also reacts with oxygen and wastes energy through photorespiration when CO2 is scarce near the enzyme. Extra CO2 shifts this balance towards carbon fixation, so the plant makes more sugar and yield rises.
- Respond to high CO2 even in low light: Photosynthesis still needs light to run its light reactions and supply energy, so this claim goes too far.
- Only CO2 is limiting: Too absolute, since water, nutrients, temperature and light all limit yield as well.
The one option that matches real plant behaviour is the rise in photosynthesis rate, because more CO2 lets RuBisCO fix carbon instead of wasting effort on photorespiration.
Let's summarize:
- Tomato and pepper are C3 crops, so CO2 enrichment reduces photorespiration and boosts carbon fixation.
- This higher photosynthesis rate is what drives the yield gain, not light independence or CO2 being the sole factor.
So option 2, the increased rate of photosynthesis at higher CO2, is the correct answer.