The correct answer is B. No changes need to be made
Step-by-step explanation:
According to grammar, a complete sentence is one that contains a subject or noun that performs the action, a main action or verb and in some cases other words that work as object or complement that explain the verb or the agents affected by it. Additionally to this, for a sentence to be complete it needs to express a complete idea, which means it does not require more words or sentences to stand and it needs to be correctly punctuated.
In the case of the sentence "No one but Marella was in the car when the accident occurred", this sentence has a subject "No one but Marella"; a verb "was" and even a complement that explains the verb "in the car when the accident occurred", also, this sentence expresses a complete idea and is correctly punctuated. Considering this, it can be concluded this sentence is already complete and therefore no changes need to be made to make it a complete sentence.