A

Step 1: Understand the principle of displacement. When the man is in the boat, both the man and the boat displace an amount of water equivalent to their combined weight.
Step 2: When the man drinks water from the pond, he is effectively removing water from the pond and adding it to his own body. This water is no longer contributing to the displacement caused by the boat.
Step 3: As he drinks the water, the total weight that the boat has to displace decreases, leading to a drop in water level since less water is pressed down by the boat.
Therefore, the water level in the pond decreases.