Answer:
d. an adverb clause modifying was
Explanation:
Adverb clauses are groups of related words containing at least a subject and a verb, and whose function is to modify adjectives, verbs or other adverbs. This type of clauses provide information about place, time, frequency, degree, condition, reason, and manner and are introduced by subordinating conjunctions like when, while, since and even though.
In the sentence, the adverb clause is “When the Little Red Schoolhouse began to give way to the consolidated school in the 1920s” because it contains all the foregoing characteristics. Besides, it modifies the verb “was” of the second clause because it provides information about time, that is, about <em>when</em> one of the problems was transportation.
