Question

Which is a complete sentence?

a. An underwater forest with large kelp for sea creatures.
b. A window into the ocean ecosystem.
c. The Old Point Loma Lighthouse, 462 feet above sea level.
d. The Bayside Trail affords spectacular views of San Diego Bay.

Answer 1

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.

Answer 2

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.