In C3 and C4 plants, primary carboxylation takes place with the help of

The correct option is RuBP carboxylase and PEP carboxylase.

In photosynthesis, primary carboxylation is the first step where inorganic carbon dioxide (CO₂) is captured and fixed into an organic molecule. This process differs between C₃ and C₄ plants because of their distinct biochemical pathways and leaf anatomy.

For C₃ plants:
The primary fixation of carbon dioxide occurs in the mesophyll cells. The CO₂ acceptor molecule is Ribulose-1,5-bisphosphate (RuBP), which is a 5-carbon compound. The enzyme catalyzing this primary carboxylation reaction is Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known as RuBP carboxylase (or Rubisco). This reaction yields two molecules of 3-phosphoglyceric acid (3-PGA), a 3-carbon compound.

For C₄ plants:
The primary fixation of carbon dioxide occurs in the mesophyll cells, but the primary CO₂ acceptor is Phosphoenolpyruvate (PEP), which is a 3-carbon compound. The enzyme catalyzing this primary carboxylation is Phosphoenolpyruvate carboxylase, commonly known as PEP carboxylase (or PEPCase). This enzyme has a high affinity for CO₂ and fixes it into oxaloacetic acid (OAA), a 4-carbon compound. (Later, CO₂ is released in the bundle sheath cells to be refixed by Rubisco in the Calvin cycle, but the primary carboxylation is catalyzed by PEP carboxylase).

Thus, the enzymes responsible for primary carboxylation in C₃ and C₄ plants are RuBP carboxylase and PEP carboxylase, respectively.