The surface water in a lake is going to freeze. Now the temperature of water at the bottom is ..

277 K means $4^{0}C$. As temperature falls below 4°C, surface water becomes lighter( less dense) compared to the water below and floats until it freezes. Once frozen, the ice insulates the water at the bottom from cooling so its temperature is maintained between 0 and almost 4°C, keeping it from freezing.