Which of the following is a C4 plant?

The correct option is Sugarcane.

To understand why sugarcane is a C4 plant, let us break down the differences in photosynthetic pathways:
1. What is a C4 plant? C4 plants are adapted to hot, dry environments. They minimize photorespiration (a wasteful process that occurs when the enzyme RuBisCO binds to oxygen instead of carbon dioxide) by using a specialized two-step carbon fixation pathway.
2. The C4 Pathway: In C4 plants, carbon dioxide is first fixed in the mesophyll cells into a 4-carbon compound (oxaloacetate) by the enzyme PEP carboxylase (which has a high affinity for carbon dioxide and no affinity for oxygen). This 4-carbon compound is then transported to bundle-sheath cells, where it is decarboxylated to release carbon dioxide directly around RuBisCO, ensuring efficient carbon fixation even under high temperatures and dry conditions.
3. Analyzing the Options:
- Wheat and Rice are classic examples of C3 plants. They use the standard C3 pathway where the first stable product of carbon fixation is a 3-carbon compound (3-phosphoglycerate), and they are prone to photorespiration in hot, dry conditions.
- Cactus is a CAM (Crassulacean Acid Metabolism) plant, which opens its stomata at night to capture carbon dioxide to minimize water loss in extremely arid environments.
- Sugarcane (along with maize, sorghum, and millet) is a tropical grass that has evolved the Kranz anatomy and the C4 pathway, making it highly efficient at photosynthesis in high temperatures and light intensities.