Which statement best describes a C3 plant?

• A. It initially fixes $C_{0}2$ with PEP carboxylase into a 4-carbon compound in mesophyll cells, then delivers $C_{0}2$ to bundle-sheath cells for the Calvin cycle.
• B. It opens stomata mainly at night and fixes $C_{0}2$ into organic acids for storage, then releases $C_{0}2$ for the Calvin cycle during the day.
• C. It performs the Calvin cycle in bundle-sheath cells only and largely avoids photorespiration by physically separating light reactions from carbon fixation.
• D. It uses Rubisco to fix $C_{0}2$ directly in mesophyll cells, producing a 3-carbon compound (3-PGA) as the first stable product of carbon fixation.

Answer: (D)

Verified step by step guidance

1. Understand that C3 plants are named for the 3-carbon compound (3-phosphoglycerate, or 3-PGA) that is the first stable product of carbon fixation in the Calvin cycle.
2. Recall that in C3 plants, the enzyme Rubisco directly fixes carbon dioxide (CO\_2) in the mesophyll cells during the Calvin cycle, without any preliminary steps involving other carbon compounds.
3. Recognize that the other options describe characteristics of C4 plants (which fix CO\_2 first into a 4-carbon compound using PEP carboxylase and then transport it to bundle-sheath cells) and CAM plants (which fix CO\_2 at night into organic acids and release it during the day).
4. Note that C3 plants do not separate the light reactions and carbon fixation spatially (like C4 plants) or temporally (like CAM plants); both processes occur in the mesophyll cells.
5. Therefore, the best description of a C3 plant is that it uses Rubisco to fix CO\_2 directly in mesophyll cells, producing a 3-carbon compound (3-PGA) as the first stable product of carbon fixation.