Every Rasool is a Nabi, but every Nabi is not a __?

Nabi

Prophet

Messenger

Imam

Every Rasool is also a Nabi but a Nabi is not a Rasool

Nabi is the correct answer because it completes the statement accurately:

every Rasool (Messenger) is a Nabi (Prophet), but not every Nabi is a Rasool.