Which of the following sentences best uses grammar for style and clarity?

Which of the following sentences best uses grammar for style and clarity?

a. When a story is interrupted to present a past event, that is what a flashback is.

b. A flashback is a device where a past event interrupts the story.

c. A flashback is a device in which a story is interrupted to present a past event.

d. Interrupting a story to present a past event is what a flashback is for.