Explanation

(c) A lake cools from the surface to bottom. Above $4℃$ the cooled water at the surface flows to the bottom because of it’s greater density. But when the surface temperature drops below $4℃$ (here it is $2℃$), the water near the surface is less dense than the warmer water below. Hence the downward flow ceases, the water at the bottom remains at $4℃$ until nearly the entire lake, is frozen