Answer: The sentence provided is a b) run-on sentence.
Step-by-step explanation: A run-on sentence represents a writing mistake that consists in connecting two or more independent clauses without including a punctuation mark or a conjunction between them. "Noah Webster supported himself through sales of his spelling book over 100 million copies of the book were sold" is a run-on sentence because the two independent clauses that form this sentence ("Noah Webster supported himself through sales of his spelling book" and "Over 100 million copies of the book were sold") have not been separated by a semicolon, a period or a conjunction.