The words in the sentence which make up the adverb phrase and the words it modifies are:
B. during the summer; modifies 50 miles a day
Adverbial phrase starts with an adverb; in this case, adverb "during" is used which tells when the action takes place. Like the function of adverbs, it modifies verbs, adverbs, and adjectives which in this sentence gives more details about the phrase "50 miles a day".
