The correct answer is A.
OBJ-5.3: Application whitelisting will only allow a program to execute if it is specifically listed in the approved exception list. All other programs are blocked from running. This makes it the BEST mitigation against a zero-day virus. An intrusion detection system might detect the anomalous activity created by a piece of malware, but it will only log or alert based on the activity, not prevent it. A host-based firewall may prevent a piece of malware from establishing a network connection with a remote server. Still, again, it wouldn’t prevent infection or prevent it from executing. An anti-malware solution is a good investment towards improving your security. Since the threat is a zero-day virus, an anti-malware solution will not detect it using its signature database.