Substrate-level phosphorylation takes place in
a. Krebs cycle and the transition reaction
b. Glycolysis and the electron transport system
c. Electron transport system and the transition reaction
d. Glycolysis and the Krebs cycle

Hint: Substrate-level phosphorylation is a metabolic process. In this, ATP is synthesized as a result of the oxidation of organic compounds. The involved organic compound serves as both electron donor and electron acceptor and the high energy phosphates bonds are transferred to ADP to form ATP.

Complete answer:
Substrate-level phosphorylation is a metabolic process in which ATP is synthesized as a result of oxidation of organic compounds. It occurs in the cytoplasm of the cell during glycolysis and mitochondria of the cell during Krebs cycle.

Glycolysis: It is the breakdown of sugars. This process takes place in the cytoplasm of the cell. It is also known as the Embden-Meyerhof pathway or EMP pathway. This process occurs aerobically as well as anaerobically and does not involve molecular oxygen. It is a process in which a sequence of ten enzyme catalyzed reactions that converts glucose into pyruvate followed by the production of ATP. In this process, one mole of glucose is partly oxidized to two moles of pyruvate. It also includes formation of lactate from pyruvate.

Krebs Cycle: It is also known as citric acid cycle. It takes place in the mitochondrial matrix present in the mitochondria. It is an aerobic reaction and hence, requires oxygen. Although, there is no direct participation of oxygen. It is the most important central pathway which connects almost all individuals of the metabolic pathway. It involves the oxidation of acetyl CoA to carbon dioxide and water. This cycle is ended by the ATP production.

Hence, the correct answer is option (D).

Note: Substrate-level phosphorylation significantly revolves around ATP. ATP, Adenosine Triphosphate is an energy carrying molecule which is found in the cells. It takes energy from the breakdown of food molecules and provides it to other cellular processes.