I don’t really understand what a adverb clause is or what it’s asking can somone explain?

Explanation:

A adverb is a dependent clause that functions as an adverb.

This means that this part of a sentence could not stand on its own a separate sentence without the independent part of the sentence.

the answer should be d) he was one of those young thousands who greeted the war with a certain amount of relief, welcoming the liberation from webs of tangled emotion" since it is the best option because it shows his desire to free himself. 

hope this answer you! have a great day.