Question

is this a complete sentence or a fragment? sheldon plankton’s big book of riddles. complete sentence fragment

Explanation:

A complete sentence needs a subject and a predicate (a verb or a verb phrase showing action or state). The phrase "Sheldon Plankton’s big book of riddles" is a noun phrase (a subject - like entity) but lacks a predicate (no verb to show what it does or is). So it's a fragment.

Answer:

Fragment