Which clause in this sentence is used as an adjective?
when beowulf arrived, the danes welcomed him, for they desperately needed someone who could stop grendel.
a. when beowulf arrived
b. the danes welcomed him
c. they desperately needed someone
d. who could stop grendel

The only sentence here used as an adjective is D) who could stop Grendel. That sentence describes the pronoun someone (not any someone, but someone who could stop Grendel), and therefore it must be an adjective.
A is an adverb clause.
B is an independent sentence.
C is also an independent sentence.