Which of the following items should be checked before operating agricultural machinery?

Checking the fuel level, coolant, engine oil, and hydraulic oil is essential before operating agricultural machinery because these items are critical for ensuring the equipment runs efficiently and safely. Each of these fluids plays a vital role in the performance and safety of the machinery.

Fuel level is necessary to ensure the machine can operate without interruption. Insufficient fuel could lead to a stall, potentially placing the operator in a hazardous situation. Coolant is crucial for regulating the engine's temperature, preventing overheating, which can cause engine damage. Engine oil lubricates the moving parts, reducing friction and wear, while also helping to keep the engine clean. Hydraulic oil is key for the operation of hydraulic systems used for various functions, such as lifting or steering, and its low level can compromise operational capability.

In contrast, focusing solely on tire pressure or transmission fluid would not provide a comprehensive safety check. While tire pressure is important for stability and traction, and transmission fluid is necessary for proper gear operation, these checks alone do not cover the full range of critical fluids needed for safe and effective machinery operation. Checking all the relevant fluids ensures the machinery is in optimal working condition before use.