This scenario matches a session fixation attack because Michael sets up a valid session identifier with the application first, then forces the victim to authenticate while using that same pre-established session. In CEH terms, session fixation occurs when an attacker "fixes" or plants a known session ID in the victim's browser, typically via a crafted URL parameter, cookie setting through a subdomain, or a redirect that preserves a session token. If the application does not regenerate the session ID after login, the victim's authentication becomes bound to the attacker-known session. The attacker can then reuse that same session ID to access the application as the victim, exactly as described when Michael "uses that session to access the portal" after the employee logs in.
The other options do not fit the mechanism. Session sniffing relies on capturing session tokens from network traffic, usually when encryption is missing or weak, but the question focuses on a link and a pre-initialized session rather than intercepting traffic. Session replay generally refers to capturing and replaying authentication exchanges or tokens, not pre-setting a session before the victim authenticates. "Session donation" is not the standard CEH label for this behavior and is not the best match to the described flow.
CEH-recommended mitigations include regenerating session IDs immediately after authentication and privilege changes, rejecting session IDs supplied in URLs, setting Secure and HttpOnly cookie flags, enforcing SameSite where appropriate, implementing short idle timeouts, and adding server-side controls to detect concurrent use or abnormal session binding changes.