Which centralized web-filtering technique categorizes websites into groups such as social networking and gambling?

The correct answer is content categorization. This technique involves organizing websites into predefined groups based on their content, such as social networking, gambling, or educational. By categorizing websites, organizations can implement policies to allow or block access to certain types of content based on their specific needs or security policies.

Content categorization is essential for effective web filtering because it allows for broader rules to be applied efficiently across many sites instead of evaluating each site individually. This method enhances both user experience and security by providing a more granular level of control over what types of websites can be accessed from the organization's network.

URL filtering, while closely related, focuses more on specific URLs rather than categorizing them into broader groups. Traffic shaping and rate limiting involve managing bandwidth and traffic rather than controlling access to content based on its category. Thus, content categorization stands out as the most relevant technique for the question posed, as it directly addresses the organization of websites into meaningful categories.