Final answer:
In terms of COMSEC, the modern key custodians are responsible for tracking expiration dates of all modern keys to prevent security incidents.
Step-by-step explanation:
In the context of COMSEC (Communications Security), the entity responsible for tracking the expiration dates of all modern keys to prevent COMSEC incidents is option d) Modern key custodians. These custodians are in charge of managing and safeguarding sensitive cryptographic keys that are used to secure communications. By monitoring expiry dates, these custodians help to maintain communication security and reduce risks of breaches or unauthorized access.
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