Input validation is the correct answer because it directly mitigates remote code execution vulnerabilities by ensuring that only properly formatted data is accepted by the web application. By validating all input, the application can prevent malicious data that could be used in an exploit from entering the system and being executed. While input validation by itself might not fully mitigate all remote code execution risks, it is the most effective single control among the given options. Patch management is important, but it does not prevent exploitation of zero-days or unpatched vulnerabilities. Firewalls and IDS/IPS are boundary defense mechanisms that do not directly address the specific coding issues that allow for remote code execution. Session management is important for user session security but does not prevent remote code execution.