Final answer:
The sentence 'The children at Boston Elementary School' is a sentence fragment because it has a subject but lacks a verb or predicate, making it unclear and incomplete.
Step-by-step explanation:
The sentence provided, "The children at Boston Elementary School," is an example of a sentence fragment. A complete sentence requires at least a subject and a predicate. In this case, the group of words has a subject ('The children') but no verb or complete thought, which means it cannot stand on its own as a sentence. The best description for the sentence is (d) the sentence is unclear about its meaning because it does not express a complete thought.