The correct solution is to create an Event Threat Detection (ETD) custom module. ETD is the Security Command Center (SCC) service designed to analyze logs for active threats, anomalies, and malicious behavior. The user's requirement is to use a list of known Indicators of Compromise (IoCs) and external signals, which directly aligns with the purpose of ETD.

In contrast, Security Health Analytics (SHA), mentioned in options A and B, is a posture management service. SHA custom modules are used to detect misconfigurations and vulnerabilities in resource settings, not to analyze log streams for threat activity based on IoCs.

Event Threat Detection provides pre-built templates for creating custom modules to simplify the detection engineering process. The "Configurable Bad IP" template is specifically designed for this exact use case. It allows an organization to upload and maintain a list of known malicious IP addresses (a common form of external IoC). ETD will then continuously scan relevant log sources, such as VPC Flow Logs, Cloud DNS logs, and Cloud NAT logs. If any activity to or from an IP address on this custom list is detected, ETD automatically generates a CONFIGURABLE_BAD_IP finding in Security Command Center for review and response. This approach is the native, efficient, and supported method for integrating IP-based IoCs into SCC, unlike option D which requires building a complex, manual pipeline.

(Reference: Google Cloud documentation, "Overview of Event Threat Detection custom modules"; "Using Event Threat Detection custom module templates")