Which of the following actions can suppliers complete in an RFP email?

The correct action that suppliers can complete in an RFP email is to mark it in progress. This capability allows suppliers to indicate that they are actively working on the RFP and have begun the process of preparing their proposal. This status update is crucial for event planners, as it provides visibility into which suppliers are engaged and how the process is moving forward.

Options such as replying to questions and contacting the event planner directly typically occur outside the RFP email functionality. They are part of the communication process that takes place during the RFP lifecycle but do not necessarily feature as tasks within the RFP tool itself. Changing the submission deadline is generally a function controlled solely by the event planner and not by the suppliers, as deadlines are set to ensure a fair and organized proposal process.