Answer:
The reader does not know why Diane is carrying a brain.
Step-by-step explanation:
Suspense or tension in a story is when the readers are unsure about what the events in the story will be like. The feeling of not knowing what will happen, along with the eagerness to see what will happen is known as suspense.
In the given passage from "A Drop in the Bucket", the reader knows that Diane's mother is in the room. This story is about Diane France, a famous forensic anthropologist who deals with bones to specify and determine their causes of death. So, when she returned to the room with the brain in the bucket, we, along with her mother, are shocked to actually see a brain in a bucket. It may be common for Diane to deal with these things but it certainly isn't for us and her mother. And the suspense is further heightened when we don't know why she is carrying a brain like it is nothing unusual.
Thus, the correct answer is the last option.