Question

Which words in the sentence are the adverb clause?

Answer 1

The answer is c. afterward to sit by the back door
An adverb clause is a word of words that modify a verb, adjective, or adverb

Answer 2

The correct answer is D, because that is the only subordinate/adverb clause among these examples. A is an adverbial phrase, not a clause - it doesn't have an active verb. B is a complete independent sentence. C cannot be a clause, because it doesn't have an active verb - 'to sit' is an infinitive, it isn't active or passive.