Part A
Meg is most upset with Mr. Jenkins because he asks Meg to accept the fact that Mr. Murry will not return.
Part B
The quotation from the text that best supports the answer to Part A is:
“Mr. Jenkins sighed. ‘Do you enjoy being the most belligerent, uncooperative child in school?’”
This statement from Mr. Jenkins indicates his frustration with Meg's behavior and suggests that he wants her to accept a situation that she is clearly struggling with, which relates to her feelings about her father's absence.