Final answer:
The missing word in Agatha Christie's title is 'Nile', making the correct title 'Death on the Nile'.
Step-by-step explanation:
The missing word in Agatha Christie's title "Death on the _____" is b) Nile. The correct title is "Death on the Nile", which is one of Christie's most famous works. This detective novel features the Belgian detective Hercule Poirot, who investigates a murder that takes place on a luxury cruise traveling down the Nile River in Egypt.