What protocol uses TCP as its transport layer and operates on port 443?

The protocol that uses TCP as its transport layer and operates on port 443 is HTTPS. HTTPS stands for HyperText Transfer Protocol Secure. It is an extension of HTTP (HyperText Transfer Protocol) but includes an additional layer of security by utilizing SSL/TLS (Secure Sockets Layer/Transport Layer Security). This security layer encrypts the data transmitted between the user's browser and the web server, ensuring confidentiality and integrity.

Using TCP as its transport layer is significant because TCP provides reliable data transmission, ensuring that packets are delivered in order and retransmitted if they are lost during transmission. Port 443 is the standard port assigned to HTTPS traffic, allowing web servers and browsers to handle secure communications effectively.

In contrast, the other protocols listed operate on different bases: HTTP uses port 80 and does not include the secure encryption aspect, FTP operates on ports 20 and 21 primarily for file transfers without encryption, and SMTP is used for sending emails and typically operates on port 25. This makes HTTPS the correct answer as it aligns with both the transport protocol (TCP) and the specified port (443).