Question

Read the sentence. The __________ mechanism made it possible for a machine gun mounted on a plane to fire between its spinning propeller blades. Which option correctly completes the sentence?

Answer 1

The answer is "interrupter". You will get a faster answer if you include the answers in your test next time.

Answer 2

The synchronization mechanism.

I believe that's the term you're looking for.  The synchronization gear was also known as a gun synchronizer, or sometimes has been referred to as an "interrupter," although that term doesn't really describe it's function accurately.  Essentially, the mechanism synchronized the timing of the airplane propeller and the firing speed of the gun so that the bullets would be flying through the space between the propeller blades as they spun.