“i have a letter somewhere” looking in the machine drawer and finding the letter in the bottom of the workbasket. “he says to tell you he will be in vera cruz the beginning of next month,”—“and if you still have the intention of joining him”—bang! clatter, clatter, bang!

I would say that the option that best explains this excerpt's purpose in the novel's plot structure is the third one: the excerpt forms the point of greatest tension and acts as the climax.

Explanation:

This excerpt shows an argument between two characters, a moment of tension, a climax. A story can have more than one moment of tension, but usually there is one moment of climax and from that moment on the tension starts falling down. We can't know based on this excerpt if there are other moments of tension, probably there are, but this one is definitely a moment of tension between the kid and the mother because they are arguing about some trip the kid wanted to go to. The excerpt's purpose is to set the moment of gratest tension.

b b b b b b   b b b   b b b b b

d understatement