To identify when an application deployed on an EC2 instance is spreading malware with the least operational effort, the best solution is to use Amazon GuardDuty. GuardDuty is a threat detection service that continuously monitors for malicious or unauthorized behavior to help protect your AWS accounts, workloads, and data. It analyzes DNS requests and VPC flow logs to detect unusual patterns that may indicate malware spreading. This solution requires minimal operational effort because GuardDuty is a managed service, meaning AWS handles the infrastructure and maintenance. Option A is correct because it directly addresses the requirement by leveraging GuardDuty's capabilities to analyze traffic patterns for signs of malware spreading. Option B is incorrect because deploying decoy systems is not a direct method for identifying malware spreading from existing EC2 instances. Option C, while effective for traffic inspection, introduces more operational effort due to the need to manage an IDS appliance. Option D is incorrect because Amazon Inspector is designed for assessing applications for vulnerabilities and deviations from best practices, not for real-time detection of malware spreading.