Final answer:
A summary or follow-up memo ensures a shared understanding among recipients about decisions made and provides a written record or recap of topics discussed in meetings, especially those not resolved.
Step-by-step explanation:
A summary or follow-up memo serves several important functions in the realm of business communications. Not only does it cover an infinite variety of topics, which may include requests for information or action, responses to inquiries, or descriptions of procedures, but it also plays a crucial role in ensuring that all recipients have a shared understanding of the decisions made during a meeting. Furthermore, it acts as a written record of the meeting or conversation, particularly useful for capturing the essence of a topic discussed in a meeting, including those that may have not been resolved at the time. In essence, the correct answer to what a summary or follow-up memo does is option 4) Both b and c, as it ensures mutual understanding and provides a record.