Implement stronger security controls on the acquired subsystem is the correct choice.

Implementing stronger security controls on the subsystem directly reduces its attack surface and raises the cost of exploitation. Hardening, secure configuration, strong authentication, encryption, and runtime protections address vulnerabilities at the asset level and provide measurable reductions in both likelihood and impact.

Controls applied to the subsystem also enable defense in depth and containment. If the vendor component contains a flaw these protections can limit privilege, isolate the component, and enable monitoring and response so that supply chain compromises have lower impact.

Reduce the number of external vendors participating in the initiative is not the best mitigation because consolidating vendors may reduce management overhead but it does not directly harden the acquired subsystem. Vendor reduction can be impractical and it will not eliminate vulnerabilities in the component itself.

Establish contractual audit rights and require vendor security attestations is useful for governance and verification but it is an administrative control. Attestations and audits depend on vendor transparency and timeliness and they do not by themselves fix or harden the subsystem.

Increase the frequency of vulnerability scanning and penetration testing for the subsystem improves detection and validation but it is primarily a discovery activity. Finding vulnerabilities is important but it does not reduce risk unless findings are promptly remediated and stronger controls are put in place.

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