Of these, the best answer is cloture, though depending on what your teacher or text means by "new Senate rules" it could be that as well.
Before 1917, the filibuster was extremely powerful because there was no way to stop it. In 1917, however, the Senate created the rule allowing for cloture to be invoked. If cloture was invoked, the filibuster was over and a vote could be taken on the actual bill.
In 1975, new Senate rules made it easier to invoke cloture. Before then, a two-thirds majority of those present was needed to invoke cloture. Now, the requirement is for "only" three-fifths of all Senators to vote for cloture. This means that a majority of 60 votes can end a filibuster.
Because of this, it is easier to break filibusters than it once was and the filibuster is not as strong of a weapon.