A user's web browser keeps redirecting to unwanted websites despite clearing the cache. What is the most likely cause?

Correct answer: Browser plug-in malware

Option C is correct because browser redirects that persist after clearing the cache are a hallmark sign of a malicious browser extension or plug-in that actively injects redirect logic into the browsing session independent of cached content. Option A, an expired SSL certificate, causes certificate warnings and broken HTTPS connections, not unsolicited redirects to other sites. Option B, a corrupted DNS cache, might cause one site to resolve to a wrong IP but would be fixed by flushing the DNS cache, not just clearing the browser cache, and would not produce ongoing redirects to attacker-controlled pages. Option D, incorrect proxy settings, could redirect traffic through an unintended proxy, but the question specifies that cache clearing did not help, making an installed malicious extension the more likely persistent culprit.