Final answer:
The reticular formation in the midbrain is responsible for maintaining wakefulness and alertness. Option D.
Step-by-step explanation:
The part of the brain that is important in keeping the forebrain alert and producing a state of wakefulness is the Reticular formation.
The reticular formation is a structure located in the midbrain, which extends up into the forebrain and down into the hindbrain. It plays a critical role in regulating the sleep/wake cycle, arousal, alertness, and motor activity.
The other options mentioned in the question, such as the hippocampus, limbic system, hindbrain, and medulla, are not directly responsible for maintaining wakefulness.
So Option D is correct.