I've heard rumours of all 3. Todd Howard did an interview where he stated that their next game would run of a heavily modified Gamebryo engine. However, Gamebryo is pretty old by now(in addition to being inefficient and buggy as hell), and one of the large criticisms for New Vegas was its outdated engine. Bethesda would have to do some heavy work to modify it enough to make TESV look like a true sequel.
They also have an exclusive (?) license to work with the id Tech 5 engine. While, apparently, this engine is good at large exteriors, Bethesda has said the opposite.
There have also been rumors of an in-house engine that will be used for TESV. Nothing more in known.

I believe they are using a heavily modified Gamebryo engine. Too bad they aren't using the id Tech 5 engine.

There are already several threads, but apparently it's in-house.

In-house engine.