I agree with the suggested answer.
The most likely cause of the issue is C. The customers are using an unsupported OS.

Reasoning:
When a load balancer is configured to support only TLSv1.3, clients (customers' browsers or operating systems) that do not support TLSv1.3 will be unable to establish a secure connection.

TLSv1.3 is the newest version of the TLS protocol and requires updated software to be supported.
Older operating systems and browsers may only support older TLS versions like TLSv1.2 or TLSv1.1.
If a client only supports older versions and the server (load balancer) only supports TLSv1.3, a connection cannot be negotiated, resulting in the customer being unable to access the website.

Reasons for eliminating other options:

A. The certificate is expired: While an expired certificate can cause website access issues, it's less likely to be the *most* likely cause immediately after a TLS version change. Expired certificates typically affect all users, not just some.
B. There is a mismatch between the key and the certificate: Like an expired certificate, a key/certificate mismatch would likely affect all users.
D. The load balancer was misconfigured: While possible, a misconfiguration is less likely than the client OS/browser incompatibility, given the scenario explicitly mentions the change to TLSv1.3. A misconfiguration would also more likely affect all users.