Application allow listing is the best mitigation technique among the given options for preventing malicious code injection as it controls which applications are permitted to run on the network. By only allowing vetted and trusted applications, this measure significantly reduces the likelihood of executing malicious code. Patching is important for addressing known vulnerabilities but won't prevent unvetted applications from running. Encryption protects data at rest and in transit, but it does not prevent the execution of malicious code. Configuration enforcement ensures systems are set up securely but also does not prevent the running of potentially harmful applications if they are not explicitly disallowed.