To block certain URLs based on the "Chat and Instant Messaging" category, the network administrator should select a reputation score of 5. A reputation score is a numerical value that indicates the likelihood of a URL being malicious or unwanted. The lower the score, the higher the risk. A score of 5 means that the URL is suspicious or potentially harmful, and should be blocked or inspected12. A score of 3 means that the URL is unknown or unverified, and may be allowed or blocked depending on the policy settings12. A score of 10 means that the URL is trustworthy or benign, and should be allowed12. A score of 1 means that the URL is malicious or high-risk, and should be blocked12. Therefore, a score of 5 is the most appropriate to block the
"Chat and Instant Messaging" category, which may contain unwanted or problematic websites. References:
* Reputation score, section "Reputation score".
* Web content filtering, section "What is web content filtering?".