Which of the following best describes a way to safely transmit Controlled Unclassified Information (CUI)?

A is the best choice because it emphasizes the importance of proper marking and authentication when transmitting Controlled Unclassified Information (CUI). Including a CUI marking in the subject header alerts the recipient to the sensitivity of the information being sent, ensuring that it is handled appropriately. Digitally signing the email further enhances security by providing assurance of the sender's identity and the integrity of the message, as it verifies that the content has not been altered during transmission.

This approach aligns with DoD guidance on protecting sensitive information and ensuring that individuals who receive it understand its classified nature and adhere to proper handling procedures. Safe transmission methods are crucial to prevent unauthorized access or data breaches, making this method the most secure option listed.