What type of dementia happens abruptly, is variable hour to hour, and is usually reversible?

The correct answer is DELIRIUM
Delirium is a temporal mental state with sudden onset; it is usually reversible. It includes symptoms such as confusion, inability to concentrate, disorientation, anxiety, and sometimes hallucinations.

Delirium can be caused by severe infection, dehydration, disorientation, and drug intoxication. Other risk factors for delirium include the history of past surgery, history of stroke and dementia, people under emotional stress, and sleep deprivation.

There are three types of delirium. This includes delirium tremens, hyperactive delirium, and hypoactive delirium. Delirium tremens is a psychosis of chronic alcoholism, usually due to alcohol withdrawal, which can be fatal.

Hyperactive delirium is characterized by being restless, agitated, and uncooperative. Hypoactive delirium is characterized by sluggishness, being inattentive, and abnormal drowsiness.