To prevent the wasteful process of photorespiration, C_4 plants...

In $C_4$ plants like sugarcane, the primary $CO_2$ acceptor is a 3-carbon molecule called phosphoenol pyruvate (PEP).

This molecule is present in the mesophyll cells.

The enzyme responsible for this initial $CO_2$ fixation is PEP carboxylase (PEPcase).

The first stable product formed is a 4-carbon compound, oxaloacetic acid (OAA).

Ribulose-1,5-bisphosphate (RuBP) is a 5-carbon molecule that acts as the primary $CO_2$ acceptor in $C_3$ plants.

Answer: Phosphoenol pyruvate