Solution:

In C 4 plants, mesophyll cells are responsible for the early fixation of Carbon dioxide or carboxylation. PEP carboxylase (or PEP case) is a Carbon dioxide fixation enzyme found in the chloroplasts of mesophyll cells. Phosphoenol pyruvate, or PEP, is the major Carbon dioxide acceptor. In the presence of PEP carboxylase (or PEP case), it reacts with Carbon dioxide to form oxalo-acetic acid or oxaloacetate (OAA). So, the correct answer is (d) ‘Oxaloacetate’.