The correct answer is: Option A. When phones had cords.
An adjective clause meets three requirements in order to serve as an adjective:
- Contains a subject and a verb.
- Starts with a relative pronoun (who, whose, which...), or a relative adverb (when, why, where...).
- Answers questions such as How many? or When did it happen?
Regarding the sentence in the example:
- Phones is the subject and had is the verb.
- Starts with the relative adverb When.
- Answers the question When did it happen?
Option A meets all requirements for an adjective clause.