What is the Senate's only defense to a filibuster?

calling a recess

invoking cloture

ending the session

ignoring the person

Letter b. invoking cloture is the answer.

In overcoming a filibuster, the senate invokes a cloture. Cloture is the only method or procedure by which the senate can vote to place a time limit on consideration over a matter or specially of a bill thereby overcoming a filibuster. The said procedure begins with the filing for cloture duly signed by 16 Senators. It is a two-step process. The vote on cloture happens in two days after the petition is filed. It is considered successful if approved by three-fifths of the full senate.

invoking cloture