What is molten rock outside earth from a volcanic eruption called? a. debris b. lava c. magma d. sediment

It is called magma when in is under the earths crust, but above the surface it is called lava. So the answer would be "B."

B: Lava
molten rock is considered Lava outside of a volcanoe because when it is in the volcanoe by the temperature being excessively hot it's called Magma. then when it leaves out of a vent in the volcanoe by it rushing down the volcanoe it is cooling down do to the amount of oxygen that is being consumed by Lava.

They both determine the exact age of rocks.