Based on the context provided, the most suitable question that Charlie can answer using the diagram of a process that occurs within a cell would be:
a) What are the inputs and outputs in the process of photosynthesis?
This question directly relates to the inputs (like carbon dioxide and water) and outputs (like glucose and oxygen) involved in photosynthesis, which would typically be represented in a diagram of that process. The other questions pertain to cellular respiration or storage of substances, which are not directly related to photosynthesis.