In schizophrenia, symptoms are broadly categorized into positive and negative symptoms based on their clinical presentation.

Positive symptoms represent an excess or distortion of normal functions. These include hallucinations, delusions, disorganized speech, and disorganized behavior. Among the options given, delusions are a classic example of positive symptoms. Delusions are fixed, false beliefs that are not amenable to reason or contradictory evidence and are a hallmark of the psychotic features of schizophrenia.

On the other hand, negative symptoms reflect a diminution or loss of normal functions. These include:
- Affective flattening (reduced emotional expression)
- Alogia (poverty of speech)
- Anhedonia (inability to experience pleasure)

Both alogia and anhedonia represent deficits in motivation, speech, or emotional response rather than added experiences.

Key point:
- Delusions are considered positive symptoms because they are an addition to normal thought processes.
- Affective flattening, alogia, and anhedonia are considered negative symptoms as they represent reductions in normal emotional and cognitive functioning.

Reference:
Kaplan & Sadock’s Comprehensive Textbook of Psychiatry, 10th Edition, Volume 1, Chapter on Schizophrenia, pp. 1274-1290