Final answer:
The adverb in the provided sentence is 'usually', which modifies the verb 'jog' to indicate how frequently Jake and Jeff exercise.
Step-by-step explanation:
In the following sentence 'Jake and Jeff are strong believers in the benefits of exercise usually jog four to five miles each morning', the adverb is 'usually'. Adverbs are words that modify or describe a verb, adjective, another adverb, determiner, clause, preposition, or sentence. They can be used to show manner, degree, place, time, frequency, degree, level of certainty, etc. In this case, 'usually' is modifying the verb 'jog', telling us how frequently Jake and Jeff jog in the morning.
Learn more about Adverb