Hi There!
A. a slab of concrete is placed over the pond and covered with ice each winter for skating (Incorrect, because if there was a slab of concrete of the ice the ice would have already melted before he even got there.)
B. bulldozers break the ice down after each winter and the pond is filled with water for the summer (Incorrect, bulldozers are way to heavy and they wold have feel in the ice the melting of ice is a natural occurrence.)
C. the fish hatch under the frozen water and their movement causes the ice to break down into water again (Incorrect, because fish eggs don't hatch when its cold.)
D. the water is frozen solid as the temperature drops each winter and the ice melts when the temperature increases again (Correct, because ice melts when the temperatures raises.)
Hope This Helps :)