Which indicators are critical during pre-flight checks?

During pre-flight checks, crucial indicators include fuel levels, oil pressure, and control surface movement. Fuel levels are essential to ensure that there is enough fuel for the planned flight, as insufficient fuel can lead to in-flight complications or emergencies. Oil pressure is vital for engine performance; it indicates that the engine is lubricated properly and is functioning within safe operational limits. Control surface movement ensures that all flight controls, such as ailerons, elevators, and rudders, are operational and respond correctly, which is fundamental for maintaining control of the aircraft during flight.

The other options involve considerations that do not pertain directly to the immediate operational integrity and safety of the aircraft for flight. Weather forecasts and altitude are relevant, but they are not physical indicators that require inspection. Departure time and snack inventory, along with passenger count and seating arrangements, are logistical aspects that, while important for flight operations, do not affect the aircraft's mechanical readiness or safety.