Final answer:
Both the assertion about the water level in the pond decreasing when a man drinks from it and the reason citing Archimedes' principle are incorrect. Archimedes' principle states that the buoyant force equals the weight of the fluid displaced, but it does not account for the change in the water level when water is ingested by a person in a boat, as the water remains part of the system.
Step-by-step explanation:
The student's question relates to Archimedes' principle, which is a fundamental concept in fluid dynamics within physics. We need to analyze the assertion and reason given in the question.
Assertion: A man sitting in a boat which is floating on a pond. If the man drinks some water from the pond, the level of the water in the pond decreases.
Reason: According to Archimede's principle the weight displaced by body is equal to the weight of the body.
The given reason is a correct statement of Archimedes' principle; it explains that the buoyant force on an object is equal to the weight of the fluid it displaces. However, this principle does not directly explain the assertion regarding the change in water level when a man drinks water from the pond. In fact, if a man drinks water from the pond while sitting in a boat, the overall level of the pond should remain unchanged because the water ingested by the man is still part of the system. Therefore, we can determine that both Assertion and Reason are incorrect in this context, which makes answer choice D correct.