Fish is to water as a is to air.
a. duck
b. bird
c. butterfly
d. sparrow

Fish is to water as a bird is to air.

Duck and Sparrow are types of birds and it's not really logical for it to be butterfly, I don't think.

Bird, because it is a general term that recognizes that birds fly in air, while fish swim in water.

Transition words are words like first, furthermore, besides, for instance, secondly, as a result using words like those should you write paragraphs showing time. you can start with first and then end with lastly or as a result