Answer: A memo
Explanation:
Of all the possible ways listed, a Memorandum would be the best way to go.
In businesses, Memos are considered a way to send information within the company about any changes or other happenings within a company. Memos can be sent to every individual or hung around the business. This way everybody can see it and be able to refer back to it. Memos also have proven overtime in the business world that they work for this purpose so using it would be best.
Face to face meetings would have helped explain the changes better but then people would not really be able to refer back to it.