A 20 year old boy c/o hearing of voices, aggressive behavior for 2 days. He has a fever for 2 days. When asked to his family, they say that he has been muttering to self and gesticulating. There is no h/o of psychiatric illness. Likely diagnosis is-
This boy is having:-

Hallucination (hearing of voices)
Aggressive behavior
Altered behavior (muttering to self and gesticulation)
Symptoms for 2 days


So, the patient is having psychotic symptoms for 2 days (less than one month) → Diagnosis is an acute and transient psychotic disorder (brief psychotic disorder).
However, there is one problem in diagnosing acute & transient psychotic disorder → This patient is having a fever. So, we can not exclude organic cause (e.g. CNS infection) as a cause of psychotic symptoms.
While the diagnostic criteria for acute & transient psychotic disorder state that there should be the absence of organic cause.
So, the diagnosis of the acute & transient psychotic disorder cannot be made without addressing the organic basis of the disease.
Actually, the best answer here would have been organic psychosis, which is not provided in the options.


Amongst the given options, Acute psychosis is the best answer because of two reasons:-

Other three options can be excluded easily.
Symptoms and duration of symptoms are meeting the criteria for acute & transient psychosis (except for the Presence of fever)

About other options 

The major symptoms in dementia are memory impairment and impairment of intellectual functions.
For Diagnosis of delirium, symptoms should be present in each one of the following areas:-


Impairment of consciousness & attention.
Global disturbance of cognition and perceptual disturbances.
Psychomotor disturbances.
Disturbance of sleep-wake cycle
Emotional disturbances

For Delusional disorder, symptoms should be there for more than 3 months.
Dementia
Acute psychosis
Delirium
Delusional disorder