Final answer:
The sergeant major's look of alarm reflects his understanding of the monkey's paw's dangerous power and the negative consequences wishes can have, something Mrs. White is unaware of.
Step-by-step explanation:
In the story The Monkey's Paw, Mrs. White's comment about wishing for four pairs of hands is an innocent yet naive remark that reflects her unawareness of the paw's sinister nature. The sergeant major's look of alarm is likely due to his experiences with the paw and the dark consequences of the wishes it grants. His reaction suggests that such wishes can have unexpected and horrifying outcomes, which Mrs. White does not comprehend at that moment.