The Krebs cycle does not occur if oxygen is not present. The correct option to the given question is option "b". This is a part of the cellular respiration in aerobic organism. The cellular respiration is named after Hans Krebs and he received a Noble prize for his work in the year 1953. This is a reaction that takes place within the mitochondria of the aerobic organisms. This is also considered as one of the earliest and oldest part of metabolism to have evolved. Sometimes it is also referred to as the citric acid cycle. this reaction helps to generate energy among the aerobic organisms present.
