D. Certificate of Incorporation

Explanation: The Certificate of Incorporation is the official document issued by the Corporate Affairs Commission (CAC) or relevant body that legally recognizes a company as having been formed. This document is crucial for a company to begin its operations as a legal entity. Other documents, like the Memorandum of Association or Articles of Association, are preparatory documents required for the incorporation process, but it is the Certificate of Incorporation that finalizes the legal formation of the company.