Which three functions of command must be completed by the first arriving fire officer during size up?

The selection of determine a strategy, select incident tactics, and establish an Incident Action Plan (IAP) reflects the essential duties of the first arriving fire officer during the critical initial size-up phase of an incident.

Determining a strategy is vital as it sets the overall direction for how the incident will be managed, identifying whether it will be an offensive or defensive approach based on the conditions encountered. Selecting incident tactics is equally important, as it involves deciding which specific actions will be taken to mitigate the incident effectively, such as fire suppression, ventilation, or search and rescue. Establishing an IAP is a critical step that formalizes the strategy and tactics into a cohesive plan that guides operations and informs all team members of their roles and responsibilities.

These functions are central to effective command and control, allowing for a rapid and coordinated response to incidents that can evolve rapidly, ensuring that resources are deployed efficiently and that the safety of personnel and civilians is prioritized. Other options do not capture the comprehensive planning and proactive decision-making needed by the first arriving officer at the scene.