To revise the sentence and to use a more formal tone the author should write it as, a loud noise startled the students.
What is startled?
Startled means to feel a sudden shock.
Because the pervious sentence, The students were totally freaked out by the loud bang, have a informal tone to it as it is using the words like freaked out and loud bang. To make it more formal the author need to use words like loud noise and startled.
Hence the correct option is, A. a loud noise startled the students.
To know more about startled:
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