Which of the following must licensees adhere to regarding their license number?

Licensees are required to include their license number on all official documents. This is important because the license number serves as a unique identifier that confirms their authority to operate within the industry. It provides transparency and accountability to clients and regulatory authorities, ensuring that transactions and communications are traceable back to a licensed professional. Including the license number on official documents helps to protect consumers by allowing them to verify the legitimacy of the licensee's credentials.

In contrast, omitting the license number from promotional material or limiting its inclusion to business correspondence does not meet the standard for transparency and professionalism expected in the industry. Using it solely as a unique identifier for clients does not encompass the broader requirement of displaying it on all official documents, meaning it would not fulfill the obligations set forth by regulatory bodies.