An important early step in cellular respiration is

The correct answer is **D. The breakdown of glucose**.

In cellular respiration, glucose is broken down to release energy, which is then used to produce ATP (adenosine triphosphate). This process begins with glycolysis, where glucose is converted into pyruvate, followed by further processing in the mitochondria, depending on whether oxygen is present (aerobic respiration) or not (anaerobic respiration).