The brainstem is a complex part of the brain, located in the back. It is constituted by many parts:
From the top:
The midbrain: which it consists of the tectum (which itself contain reticular formations), the tegmentum, and the ventral tegmentum.
The pons: which link between the medulla oblongata and the midbrain.
Medulla oblongata.
So the right answer is that It is constituted by the medulla, pons, and ventricular formation.