At what points during the year are fish—which need a large amount of dissolved oxygen—most likely to be evenly distributed throughout the water of a lake that freezes over in winter?
spring and fall when warm and cold water mixes
fall and winter when the water is warmer
summer and winter when warm water settles on the bottom
spring and summer when the cold water settles on top

The correct answer is (a)Spring and fall when warm and cold water mixes

Spring and fall when the warm and the cold water mixes and the mixing of water evenly distributes the oxygen present in lake. During winter the cold water is below the ice and the different areas of lake have different concentration of dissolved oxygen. In summer the warm water lies above the cold water, here also the concentration of dissolved oxygen varies.

Hence, during spring and fall the warm water and cold water mixes which evenly distributes the dissolved oxygen present in lake.

I think it is "Spring and Fall when warm and cold water mix"

Explanation: