The correct service for retrieving short-term, privileged credentials is the Security Token Service, which is often used in conjunction with federated user authentication or for assuming roles. These credentials are constrained by time and the permissions defined in their associated policies, fostering a secure environment that adheres to the principle of least privilege. The Security Token Service is a component of the overarching identity management system provided by AWS but specializes in this temporary credential issuance. In comparison, Cognito is mostly oriented towards user identity and access management in apps, whereas the Key Management Service is involved in creating and controlling encryption keys, not in issuing temporary security credentials based on permission policies.