Final answer:
A filibuster in the U.S. Senate can be stopped when three-fifths of the Senate votes for cloture, which is option 'D) cloture.' Sixty senators must support the cloture motion to end debate and proceed to a vote on the legislation.
Step-by-step explanation:
A filibuster can be stopped when three-fifths of the Senate votes for cloture. The option that correctly completes the statement is 'D) cloture.' In the United States Senate, the filibuster refers to the practice of speaking indefinitely to delay a vote on a piece of legislation. A filibuster can only be ended if a supermajority of senators, specifically 60 out of 100, vote in favor of a cloture motion. This is a way to cease debate and allow the final vote on a bill to proceed. The rule requiring a supermajority for cloture ensures that a minority cannot stop the Senate from conducting its business, although it does effectively raise the threshold for the passage of most legislation to 60 votes instead of a simple majority.