Final answer:
During a monthly lift deck inspection, the hydraulic fluid level should be checked to ensure proper operation and prevent damage to the hydraulic system.
Step-by-step explanation:
During the monthly lift deck inspection, the item that must be checked from the provided options is a. Hydraulic fluid level. The hydraulic fluid is essential for the operation of the lift deck, as it enables the smooth movement of components that rely on hydraulic power. Checking the hydraulic fluid level ensures that there is enough fluid to operate the lift deck without risking damage to the hydraulic system due to low fluid levels. On the other hand, options b, c, and d (engine horsepower, tire pressure, and windshield wiper speed) are not relevant to the inspection of a lift deck's functionality.