B is incorrect because fish take in oxygen constantly to respirate. If they only took in oxygen when it was dark they would die.
C is incorrect because expelling CO2 is also a constant process as a part of the reactions that take part inside living organisms, so it happens constantly (a bit like B)
D is incorrect because plants need light for food (photosynthesis) and would most likely die or have stunted growth if left in the dark.
A is correct because when it is dark, fish rely on other senses to find food as most fish cannot see as well in the dark, therefore when they have light they can see their food more easily.
