The number of ATP formed directly by a single Krebs cycle is:

Explanation

The Krebs cycle, also known as the citric acid cycle, occurs in the mitochondria and is a critical part of cellular respiration. During each cycle, it directly produces one ATP (or GTP, which is energetically equivalent to ATP). Since the complete oxidation of one glucose molecule requires two turns of the Krebs cycle, two ATP molecules are formed per glucose molecule. The other options refer to the total ATP yield from the entire process of cellular respiration, not just the Krebs cycle. Thus, the correct answer is that a single Krebs cycle directly forms one ATP.