The complete adjective clause is B. to whom I dedicated my essay about Spain.
An adjective clause is recognized as such when it completes these three requirements.
1) contains a subject and a verb
2) starts with a relative pronoun (who, whom, whose, that, or which) or relative adverb (when, where, or why)
3) it functions as an adjective. Answering one of the questions which one? what kind? how many?
1) subject - I ; verb - dedicated
2) relative pronoun : to whom
3) which one? this person