What is a self propelled missile usually launched by a ship or submarine?

The answer to this multiple-choice question was a torpedo. The reason that the question specifies "usually" is that ASW aircraft can launch torpedoes.

(The Word missile in military usage always means self-propelled weapons.)

The ASROC is a rocket-boosted torpedo, and many other types of missiles are launched from ships and submarines, including cruise missiles such as the Tomahawk. Trident (FBM) submarines can launch strategic ICBMs.

*All submarine weapons are actually ejected from the vessel prior to its activation: torpedoes are ejected by water impulse, as are SUBROC, Harpoon, and Tomahawk missiles. Their motors start once they've cleared a certain range from the vessel. Ballistic missiles are ejected by air impulse - launch is near the surface, and the missile is ejected into a pocket of air or steam where the rocket motor ignites.