A: Irony
<span>
Irony's definition: the expression of one's meaning by using language that normally
signifies the opposite, typically for humorous or emphatic effect.</span>
Speech balloons, also known as speech bubbles, dialogue balloons or word balloons, are a graphic convention used most commonly in comic books, comics and cartoons to allow words to be understood as representing the speech or thoughts of a given character in the story or events of the graphic novel or Comic.
According to the information presented before I would conclude that the main purpose of a speech bubble in a graphic novel is:
D) It illustrates internal thoughts.
idk what the sentence above is, but it's probably C) Several cities with large populations have many asthma-related health problems.