It takes a noun and a verb to make a complete sentence. There isn't a single verb in a), b), or c). "Affords" is the verb (predicate) in d)., the only complete sentence.
A complete sentence must have a subject, noun, adjective, a verb, etc. The only one that has that is D. The Bayside Trail affords spectacular views of San Diego Bay. The Bayside Trail being the subject. San Diego Bay being a noun. Spectacular is an adjective. Affords being a verb.
