Question

Which is a complete sentence?

Answer 1

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.
 

Answer 2

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.