Identify the C₄ acid among the provided options.

All of the above

The correct answer is Option (4)- All of the above

In C₄ plants, the initial CO₂ fixation occurs in mesophyll cells, where a 3-carbon molecule called phosphoenolpyruvate (PEP) serves as the primary CO₂ acceptor. This fixation is catalyzed by the enzyme PEP carboxylase (PEPcase). It's important to note that mesophyll cells do not contain the RuBisCO enzyme. In these cells, the C₄ acid oxaloacetic acid (OAA) is formed from PEP, and it subsequently gives rise to other 4-carbon compounds like malic acid or aspartic acid.

These C₄ acids are transported from mesophyll cells to bundle sheath cells. Inside the bundle sheath cells, these C4 acids are broken down to release CO₂ and a 3-carbon molecule. This 3-carbon molecule is then transported back to the mesophyll cells, where it is converted back into PEP, completing the cycle.

The CO₂ released in the bundle sheath cells enters the C₃ or Calvin pathway, which is a common pathway found in all plants. Bundle sheath cells are rich in an enzyme called Ribulose bisphosphate carboxylase-oxygenase (RuBisCO), but they lack PEPcase. Thus, the fundamental pathway leading to sugar formation, the Calvin pathway, is shared between C₃ and C₄ plants.