Final answer:
Option B corrects the comma splice in the original sentence by using the relative pronoun 'who' to connect the two independent clauses smoothly.
Step-by-step explanation:
The correct sentence is: The team's greatest rivals were the original New York Celtics, who were considered the best basketball team in the world. This correction is needed because the original sentence contains a comma splice, which is an error that occurs when two independent clauses are joined only by a comma without appropriate conjunction or punctuation. Option B fixes the error by using a relative pronoun 'who' to connect the two clauses smoothly.