What two tasks begin immediately after positions have been assigned?

The correct answer highlights the critical tasks of clearing fields of fire and preparing fighting positions, which are essential steps in establishing an effective defensive posture. Once positions are assigned, the immediate focus is on enhancing the effectiveness and survivability of the unit. Clearing fields of fire involves removing obstructions that could hinder visibility and firing capabilities, allowing Marines to engage the enemy effectively.

Preparing fighting positions entails fortifying and organizing the assigned areas to ensure maximum protection and combat effectiveness. This preparation not only helps in defending against potential enemy attacks but also optimizes the unit's ability to counter-attack or maneuver if necessary. These tasks are foundational elements of military training and essential for operational success in combat situations.

While other options present relevant activities, they do not address the immediate needs for defense that come right after positions are assigned. Activities like setting up supplies or conducting reconnaissance are important, but they follow the critical initialization of securing and preparing combat positions.