An adjunct is a word, phrase or clause that can be eliminated from the sentence, without affecting it grammatically. The sentence would still make sense without an adjunct. Adjuncts are usually adverbs, adverb phrases or adverb clauses. The clause in D is an adverb clause of purpose, without which the sentence "I had to skip a year of college" is still grammatically complete. Hence D is correct. "I" is a subject, "had to skip" is a verb phrase, "a year of college" is a noun phrase that performs as the object. Upon omitting any of these parts, the sentence is grammatically incomplete, hence options A, B and C are incorrect.