As part of a zero-trust environment, Quentin is given rights that he needs only when he needs them through a checkout process and they are then removed when he is done. What mitigation technique best describes this solution?

This is an example of a least privilege implementation where only the privileges required are issued. The checkout process is a modern addition to least privilege environments where even privileges needed are only issued temporarily, making least privilege even more secure. Segmentation and isolation are used to separate systems or environments, and configuration enforcement is used to ensure that configurations continue to be set as expected.