Comprehensive and Detailed Explanation (ECIH-aligned):

Cross-Site Scripting attacks occur when untrusted input is processed and rendered by an application without proper validation or encoding. ECIH web application security guidance identifies input validation and output encoding as the most effective primary defense against XSS.

Option B is correct because sanitizing and validating all user input ensures that malicious scripts are neutralized before being stored or rendered. This addresses the root cause of XSS vulnerabilities.

Option A is a detection control, not a prevention mechanism. Option C improves system security but does not prevent application-level XSS flaws. Option D addresses abuse and DoS scenarios, not script injection.

Therefore, focusing on input validation is the most effective proactive control against XSS, as emphasized in the ECIH curriculum.