Which port must be open for secure web traffic?

Secure web traffic is transmitted over HTTPS, which utilizes port 443. This port is designated for secure communications on the internet, typically for web-based transactions that require encryption to protect sensitive data. The use of HTTPS and port 443 ensures that information exchanged between a user's browser and a web server is encrypted, enhancing the security of online activities such as banking, shopping, and any other actions that require confidentiality.

The other ports mentioned serve different purposes: port 80 is used for standard HTTP traffic, which is not secure; port 8080 is often used as an alternative to port 80 for HTTP traffic or for proxy servers but does not provide encryption; and port 21 is used for FTP (File Transfer Protocol), which is also not secure and typically transfers files without encryption. Thus, port 443 is the correct answer for enabling secure web traffic.