How did Billy's father-in-law die? (Slaughterhouse)

Airplane crash.

Explanation:

Billy knows the airplane will crash, but he says nothing. The passengers are entertained by a barbershop quartet. The group sings lewd, racist songs to entertain Billy's father-in-law. When the plane crashes into Sugarbush Mountain, Vermont, everyone is killed except Billy and the copilot.

Moments after he predicts his own death and closes his speech with the words “Farewell, hello, farewell, hello,” Billy is killed by an assassin's high-powered laser gun. He experiences the violet nothingness of death, and then he swings back into life and to early 1945.

Explanation:

because

i would have to say "a" or "d".