Which action should a SysOps administrator take to resolve this issue?

Explanations:

Configuring the CloudFront distribution behavior to forward the User-Agent header allows CloudFront to pass the User-Agent information from the client to the ALB. This enables the ALB to serve the appropriate version of the website (desktop or mobile) based on the user’s device type.

Adding a User-Agent header to the list of origin custom headers in the CloudFront distribution origin settings does not resolve the issue. This is because it does not forward the actual User-Agent value from the incoming request, but rather uses a fixed value, which won’t allow dynamic handling based on the user’s device.

Enabling IPv6 on the ALB and updating the CloudFront distribution origin settings to use the dualstack endpoint does not address the root cause of the problem, which is the User-Agent handling. This option would only improve network compatibility for IPv6 users but would not affect which version of the website is served.

Enabling IPv6 on the CloudFront distribution and updating the Route 53 record to use the dualstack endpoint does not address the problem with serving the correct version of the website. The User-Agent handling remains unaddressed, and this option primarily affects IP addressing rather than content delivery based on device type.