Final answer:
The adverb in the sentence is 'promptly'.
Step-by-step explanation:
The adverb in the sentence is promptly. An adverb modifies a verb, adjective, or other adverb and indicates when, where, how, or to what extent an action happens. In this case, 'promptly' modifies the verb 'arrive' by indicating the specific manner in which Reiko wants the student to arrive - in a timely manner. Examples of other adverbs include: 'quickly,' 'slowly,' and 'carefully.'
Learn more about Adverbs