The correct option is A that cars were flying and people had wings
An adjective clause is a group of words that contain a subject and a
verb, and it functions as an adjective that modifies a noun or a
pronoun. Adjective clauses usually begin with relative pronouns such as
who, whom, whose or a relative adverb like when, where and so on.
Option A is correct because it begins with a relative pronoun 'that' that
performs as the subject and the verb "were flying". It modifies the noun
"dream".
Option B is incorrect because the clause has changed in meaning.
Option C has a clause that has changed in meaning. Hence C is incorrect.
Option D is incorrect because the answer lies in A.