Which of the following should be used to best mitigate this type of attack?"

Secure cookies are designed to protect session data from being intercepted in transit, particularly over HTTPS. While they enhance security by ensuring that cookies are only sent over secure connections, they do not address the underlying issue of input validation. Therefore, secure cookies would not mitigate the attack described in the logs.

Static code analysis is a method used to analyze source code for vulnerabilities before deployment. Although it can help identify potential security flaws in the code, it does not directly prevent attacks that exploit already deployed applications. Thus, it is not an immediate solution for mitigating the type of injection attack evident in the log entry.

Sandboxing involves executing code in a controlled environment to prevent it from affecting the host system. While sandboxing can contain the effects of a successful attack, it does not prevent the attack from occurring in the first place. Therefore, it is not an effective primary defense against the type of input injection demonstrated in this situation.