Final answer:
The complete adverb clause in the sentence "The cassette tape is older than the DVD" is "than the DVD."
Step-by-step explanation:
The complete adverb clause in the sentence "The cassette tape is older than the DVD" is "than the DVD." The adverb clause is modifying the adjective "older" and it is providing a comparison between the cassette tape and the DVD. Adverb clauses can modify verbs, adjectives, or other adverbs, and they often begin with subordinating conjunctions like "than."