**Core Concept**
The concept of partial agonism at dopamine D2 receptors is crucial in understanding the pharmacodynamics of antipsychotic medications. Dopamine D2 receptor partial agonism is a mechanism of action that offers a distinct therapeutic profile compared to full agonism or antagonism. This property is particularly relevant in the treatment of schizophrenia and other psychiatric disorders.

**Why the Correct Answer is Right**
Aripiprazole is a partial agonist at dopamine D2 receptors, which means it binds to the receptor and activates it to some extent, but not to the same extent as a full agonist. This partial agonist activity results in a stabilizing effect on the dopaminergic system, which is thought to contribute to its therapeutic efficacy in treating schizophrenia and bipolar disorder. Aripiprazole's partial agonism at D2 receptors also reduces the risk of extrapyramidal side effects (EPS) associated with full D2 receptor antagonism.

**Why Each Wrong Option is Incorrect**
**Option A:** Haloperidol is a typical antipsychotic that acts as a full antagonist at dopamine D2 receptors, which can lead to EPS and other side effects.
**Option B:** Risperidone is an atypical antipsychotic that acts primarily as a serotonin 5-HT2A receptor antagonist, with some dopamine D2 receptor antagonism, but it is not a partial agonist.
**Option C:** Olanzapine is another atypical antipsychotic that has a unique mechanism of action, involving a combination of serotonin and dopamine receptor antagonism, but it is not a partial agonist at D2 receptors.

**Clinical Pearl / High-Yield Fact**
Aripiprazole's partial agonist activity at D2 receptors makes it a useful option for patients who are at high risk of developing EPS or other side effects associated with full D2 receptor antagonism.

**Correct Answer: C. Aripiprazole**