The citric acid cycle occurs in the:

a. Golgi apparatus

b. Thylakoid lumen

c. Mitochondrial matrix

d. Across the inner membrane of the mitochondria

The correct answer will be option-C.

Explanation:

Cellular respiration is a slow process which breakdown the food components via oxidation to convert them into energy molecules. The respiration proceeds in four stages: Glycolysis, pyruvate decarboxylation, citric acid cycle and electron transport chain.

All these processes take place in different compartments of the cell-like glycolysis takes place in the cytoplasm and citric acid cycle or Kreb cycle takes place in the matrix of the mitochondria. The matrix of the mitochondria provides the necessary environment to proceed the step of the respiration.

Thus, option - C is the correct answer.