Answer:
A. Astonishing.
Step-by-step explanation:
In the given sentence, the underlined word is "staggering". This word is an adjective, meaning it modifies a noun.
As used in the given passage, the word "staggering" is used alongside "number of people". It is used to emphasize the number of people that the person hurt/ inflicted. This means that he killed a really great, astonishing, shocking number of people.
Thus, the correct answer is option A.