Which of the following metabolites is common to the respiration-mediated breakdown of fats, carbohydrates, and proteins?

During aerobic respiration, carbohydrates are converted to pyruvate and then to Acetyl CoA. Fats are broken down into glycerol and fatty acids; fatty acids undergo $\beta$ -oxidation to form Acetyl CoA. Proteins are broken down into amino acids, which can enter the cycle at various stages, primarily as Acetyl CoA. Thus, Acetyl CoA is the common entry point into the Krebs cycle for all three.