What is the MOST likely cause for a certificate warning on a public website accessed over HTTPS?

A certificate warning on a public website accessed over HTTPS is most commonly caused when the SSL certificate is not issued by a trusted root Certificate Authority (CA). Web browsers rely on a predefined list of trusted root CAs to verify the authenticity of an SSL certificate. When a website presents a certificate that cannot be traced back to one of those trusted CAs, the browser cannot guarantee that the connection is secure or that the site is who it claims to be, resulting in a warning.

The presence of this warning serves as a critical alert to users that they may be at risk of connecting to a fraudulent site, jeopardizing their personal data and privacy. Users are therefore strongly advised to approach such warnings with caution.

In contrast, other options pertain to specific aspects of certificate management. For example, while a missing private key or public key can impact the ability to establish a secure connection, these situations typically do not result in a browser warning specifically related to trust. Instead, the SSL certificate's validity and the chain of trust are paramount. Errors involving intermediate CAs can also cause trust issues, but the most decisive factor usually remains whether the certificate is issued by a recognized trusted root CA.