Question

a rock displaces 1200 L of water. The volume of the rock is?

Answer 1

This is, I guess you can call, the concept of displacement. The rock will also have a volume of 1200 L, which isn't necessarily a unit of volume for solids.

Answer 2

http://en.wikipedia.org/wiki/Displacement_(fluid)

Answer 3

Because rock sinks in water ,so you won't use Archimedes' principle.
thus it is just the volume of water displaced as iPwnBunnies said.