Question

which is a sentence fragment? the next - door neighbors of my elderly grandparents. electric eels stun prey with an intense electrical charge.

Explanation:

A sentence fragment lacks a subject - verb combination to express a complete thought. "The next - door neighbors of my elderly grandparents" is just a noun phrase, lacking a verb. "Electric eels stun prey with an intense electrical charge" is a complete sentence with a subject "electric eels" and a verb "stun".

Answer:

The next - door neighbors of my elderly grandparents.