Final answer:
The ATP/ADP cycle involves the conversion of ATP into ADP when a phosphate is added, and the conversion of ADP back into ATP when a phosphate is removed. These processes release and require energy respectively.
Step-by-step explanation:
The correct answer is C. ATP becomes ADP when a phosphate is added; ADP becomes ATP when a phosphate is removed. ATP (adenosine triphosphate) is a molecule that stores and provides energy for cellular processes. It can be converted into ADP (adenosine diphosphate) when a phosphate group is removed, releasing energy. ADP can then be converted back into ATP when a phosphate group is added, requiring energy.