iOS Security Response 16.5.1 (c)

If you read the bottom of the page it clearly says that (c) includes the content of (a).

Rapid Security Responses iOS 16.5.1 (c) and iPadOS 16.5.1 (c) include the security content of Rapid Security Responses iOS 16.5.1 (a) and iPadOS 16.5.1 (a) and fix an issue that prevents some websites from displaying properly.

About the security content of Rapid Security Responses for iOS 16.5.1 and iPadOS 16.5.1 - Apple Support

You can read it here, it is responding to an issue in webkit