In $C_4$​ plants, $CO_2$​ combines with PEP in the presence of
A)PEP carboxylase
B)RuBP carboxylase
C)RuBP oxygenase
D)Hydrogenase

Hint: The plants adapt 3 different types of pathways to fix carbon of carbon dioxide. These are $C_3$, $C_4$ and CAM. The removal of carbon from ambient carbon dioxide and its transformation into organic molecules such as carbohydrates is called carbon fixation.

Complete answer:
As an enzyme present in all multicellular plants, phosphoenolpyruvate carboxylase catalyzes the synthesis of oxaloacetate from phosphoenolpyruvate (PEP) and hydrocarbonate ions. $C_4$ plants are able to harness the reaction in order to absorb and concentrate carbon of carbon dioxide into sheath cells of a photosynthetic bundle. The greater efficiency of CO2 fixation by PEP-carboxylase is the explanation for the poor compensation value for $C_4$ plants. $C_4$ plants are found in dry and humid conditions which include plants like maize, sorghum, millet, and sugar cane, as well as tropical savanna grasses and sedges, which are valuable human food crops.
PEP carboxylase plays a crucial function in connecting $CO_2$ to the mesophyll tissue to form oxaloacetate. For species living in arid environments, the CAM cycle is normal. The plants are able to absorb $CO_2$ during night time by fixing with PEP so as to form oxalate by enzyme PEP carboxylase.

Hence, the correct answer is option (A)

Note: The greatest difference between $C_4$ and CAM plants is the way they mitigate the absence of water. $C_4$ plants migrate the $CO_2$ molecules to decrease photorespiration, while CAM plants choose when to extract $CO_2$ from the atmosphere. Photorespiration is a mechanism that happens in plants in which oxygen is added to RuBP instead of $CO_2$ .