Answer:
promptly.
Explanation:
An adverb can be defined as a word that is used in English language to modify a verb, adjective, or another adverb. Some examples are slowly, quickly, brightly, sadly, promptly, etc.
Generally, adverbs are formed by adding the suffix "ly" to the end of a verb e.g prompt + ly = promptly.
Promptly simply means to be punctual or do something quickly and soon.
Hence, the adverb in the sentence "Reiko told me to arrive promptly at 8 p.m. in order to be on time for the dinner." is promptly.
Additionally, there are six (6) main types of adverbs and these includes;
I. Adverb of time.
II. Adverb of frequency.
III. Adverb of place.
IV. Adverb of manner.
V. Adverb of reason.
VI. Adverb of intensity.
