Correct Option: D. Checking of the oil level in the engine. Explanation of the Correct Answer: Checking the oil level in the engine is a crucial daily maintenance task for tractors and other machinery. Here’s why this practice is essential:

1. Lubrication: The engine oil lubricates the moving parts of the engine, reducing friction and wear. If the oil level is too low, it can lead to inadequate lubrication, which can cause severe engine damage.
2. Cooling: Engine oil also helps in cooling the engine by carrying heat away from the moving parts. Low oil levels can lead to overheating, which can result in engine failure.
3. Contaminant Removal: Oil helps to trap contaminants and particles that can cause wear and tear. Regularly checking the oil level ensures that the oil is at an optimal level to perform this function effectively.
4. Preventive Maintenance: Daily checks can help identify leaks or other issues early, allowing for timely repairs and preventing more significant problems down the line.
5. Operational Safety: Ensuring that the oil level is adequate is part of maintaining the overall safety and reliability of the tractor during operation.

Why the Other Options are Incorrect or Weaker: A. Draining of the gear box oil. - Frequency: Draining the gearbox oil is not a daily task. This maintenance is typically performed at longer intervals, such as every few months or after a certain number of operating hours, depending on the manufacturer's recommendations. - Purpose: While it is important to maintain the gearbox oil, daily checks are not necessary as the oil does not degrade or become contaminated as quickly as engine oil. B. Removing and cleaning of engine's air filter. - Frequency: Cleaning or replacing the air filter is also not a daily requirement. This task is usually done based on the operating conditions and the manufacturer's guidelines, which may suggest checking it weekly or monthly. - Impact: While a clean air filter is essential for engine performance, it does not need to be addressed daily unless operating in extremely dusty conditions. C. Replacing of the fuel filter element. - Frequency: Replacing the fuel filter is not a daily maintenance task. This is typically done at longer intervals, such as every few hundred hours of operation or as specified by the manufacturer. - Importance: While a clean fuel filter is vital for engine performance, it does not require daily attention, making it less critical than checking the oil level. Summary of Key Points:

• Daily Maintenance: Checking the oil level in the engine is essential for lubrication, cooling, and contaminant removal.
• Preventive Action: Regular checks can prevent severe engine damage and ensure operational safety.
• Other Options: Draining gearbox oil, cleaning air filters, and replacing fuel filters are important but not daily tasks.
• Best Practice: Always refer to the manufacturer's maintenance schedule for specific tasks and intervals.

By understanding the importance of daily maintenance tasks like checking the oil level, you can ensure the longevity and reliability of agricultural machinery.