What does the Secretary of Defense (SECDEF) approve and transmit to announce the selected course of action?

The Alert Order (ALERTORD) is the correct choice because it serves as the formal expression of the Secretary of Defense's decision regarding the selected course of action. When the SECDEF approves a specific course of action in response to a situation, the ALERTORD provides a clear and concise directive to the military branches and other involved agencies, ensuring they are informed of the decisions made and the necessary actions to take.

The ALERTORD typically outlines the initial phase of a military response, which often involves readiness measures and preparatory actions. This order is essential for conveying urgency and ensuring that forces are prepared to deploy or respond as directed.

In contrast, the other options represent different aspects of military orders and planning but do not specifically relate to the SECDEF's announcement of the selected course of action. The implementation order is about executing a specific plan, the deployment directive focuses on the movement of forces, and the operational plan encompasses broader strategies and details of a military operation rather than the immediate announcement of the SECDEF's decision.