What is the purpose of a monologue?

In essence, it's simply to give more information about the particular character or plot, to make the plot or character clearer by giving his/her thoughts or backstory in the case of a plot.

halibut, flounder, tuna, and trout are all fish. so your first answer is 1.) fish. and since fish is edible, your next answer would be 3). edible

to describe those fishes more thoroughly, some of those fishes live in fresh water, and some of the fishes live in salt water, and that is not a thing that they all have in common. the question is asking what is common with all of the fish, so answer choices 2). fresh water and 4). salt water are not right.