**Core Concept**
Resperidone, a second-generation antipsychotic, is primarily used to treat psychiatric disorders by modulating dopamine and serotonin receptors in the brain.

**Why the Correct Answer is Right**
Resperidone is most commonly used to treat schizophrenia, a chronic mental disorder characterized by hallucinations, delusions, and disorganized thinking. It acts by blocking dopamine D2 receptors, which are overactive in schizophrenia patients. This blockade helps to reduce psychotic symptoms and improve cognitive function. Resperidone also has a relatively favorable side effect profile compared to first-generation antipsychotics.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because risperidone is not primarily used to treat unipolar depression. Although it may be used off-label for this condition, its primary indication is schizophrenia.
**Option B:** This option is incorrect because risperidone is not typically used to treat bipolar disorder. While it may be used as an adjunct therapy, its primary use is in schizophrenia.
**Option C:** This option is incorrect because risperidone is not primarily used to treat attention deficit hyperactivity disorder (ADHD). Although it may be used off-label for this condition, its primary indication is schizophrenia.

**Clinical Pearl / High-Yield Fact**
Risperidone is often used as a first-line treatment for schizophrenia due to its efficacy and relatively favorable side effect profile compared to other antipsychotics.

**Correct Answer: C. Schizophrenia**