Which organization issues a G-Series order to activate and assign a unit?

A G-Series order is a specific type of order within the military framework that is used to activate and assign a unit to duty status. This authority primarily resides with the Major Command (MAJCOM), which is responsible for overseeing the operational readiness, administration, and deployment of units under its command. When a Major Command issues a G-Series order, it ensures that units are properly activated in accordance with mission requirements and organizational structure, effectively managing personnel and resources for military operations.

While other entities like the Department of Defense, the Joint Chiefs of Staff, and Strategic Command play significant roles in the overarching military command structure, they do not specifically issue G-Series orders. The Major Command is the key player in this regard, making it the correct answer to the question.