What is the backup energy molecule that can be rapidly converted to ATP in active skeletal muscle?
The answer would be D. Phosphocreatine, because phosphocreatine plays a major role on energetic homeostasis in both active skeletal and cardiac muscles. Phosphocreatine is basically creatine but phosphorylated, and that is why it has such a name. It has the role of turning ADP (adenosine diphosphate) into ATP (adenosine triphosphate).
Remember, ATP is the currency of life! That is what my biology teacher taught me.