To securely receive mail that is synchronized across multiple devices, the best protocol to use is IMAP (Internet Message Access Protocol), which allows users to access and manage their emails from different clients and locations12.
IMAP uses port 143 for plain text communication and port 993 for encrypted communication using SSL/TLS12. Port 993 is the recommended port for IMAP, as it provides security and privacy for the email messages12.
The other ports in the question are not suitable for secure mail synchronization across multiple devices. Port 25 is used for SMTP (Simple Mail Transfer Protocol), which is mainly for sending mail, not receiving12. Port 110 is used for POP3 (Post Office Protocol version 3), which is an older protocol that downloads mail from the server to the client, but does not keep them synchronized12. Port 443 is used for HTTPS (Hypertext Transfer Protocol Secure), which is a web protocol, not an email protocol3.
Therefore, the correct answer is D. 993, as it is the port for secure IMAP communication, which enables mail synchronization across multiple devices.
Reference:
1: What Are Email Protocols - POP3, SMTP and IMAP
2: SMTP Ports - Secure SSL Email Ports (25,465,587 & 2525)
3: Network ports for clients and mail flow in Exchange