Answer:
C.Intrusion detection/intrusion prevention systems (IDSs/IPSs)
Step-by-step explanation:
Since the problem occurred at the server-side, it would be irrelevant to update the on-premises antivirus program. However, an IDS or IPS would be able to detect suspicious and malicious activity in the whole network system, by examining the network traffic that usually indicates the path of a Trojan or other virus.