Which of the following pathways of carbon fixation has phosphoenol pyruvate as a primary CO₂ acceptor?

The correct option is C₄ plants.

To understand why phosphoenolpyruvate (PEP) serves as the primary CO₂ acceptor in C₄ plants, we can break down the process of carbon fixation in different plant groups:

1. C₃ Plants:
In C₃ plants, the first step of carbon fixation occurs via the Calvin cycle. The primary CO₂ acceptor is a 5-carbon sugar called ribulose-1,5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme RuBP carboxylase-oxygenase (Rubisco), yielding two molecules of 3-phosphoglyceric acid (3-PGA), which is a 3-carbon compound.

2. C₄ Plants:
C₄ plants have evolved a mechanism to concentrate CO₂ around Rubisco to minimize photorespiration. This pathway, known as the Hatch-Slack pathway, occurs across two types of cells: mesophyll cells and bundle sheath cells.
In the mesophyll cells, the primary CO₂ acceptor is phosphoenolpyruvate (PEP), a 3-carbon compound. The enzyme PEP carboxylase (PEPcase) catalyzes the addition of CO₂ (in the form of bicarbonate) to PEP:

$\text{Phosphoenolpyruvate (PEP)}+{\text{CO}}_2\to \text{Oxaloacetic acid (OAA)}$

This reaction forms oxaloacetic acid (OAA), a 4-carbon acid, which gives C₄ plants their name.

3. C₂ Plants (Photorespiration):
C₂ cycle or photorespiration is a wasteful pathway that occurs when Rubisco binds to oxygen instead of carbon dioxide, and it does not involve PEP as a primary CO₂ acceptor.

Therefore, phosphoenolpyruvate is the primary CO₂ acceptor specifically in the carbon fixation pathway of C₄ plants.