**Core Concept**
The question tests the knowledge of pharmacological treatment options for Obsessive-Compulsive Disorder (OCD), a common psychiatric condition characterized by recurrent, intrusive thoughts (obsessions) and repetitive behaviors (compulsions). Selective serotonin reuptake inhibitors (SSRIs) are a first-line treatment for OCD due to their efficacy in reducing symptoms of the disorder.

**Why the Correct Answer is Right**
The correct answer, Fluoxetine, is an SSRI that works by selectively inhibiting the reuptake of serotonin (5-HT) in the synaptic cleft, thereby increasing the concentration of serotonin available for binding to postsynaptic receptors. This leads to a decrease in the symptoms of OCD. Fluoxetine has been extensively studied in clinical trials and has been shown to be effective in reducing the severity of OCD symptoms in a significant proportion of patients.

**Why Each Wrong Option is Incorrect**
**Option A:** Clomipramine - While clomipramine is a tricyclic antidepressant (TCA) with some serotonin reuptake inhibitory activity, it is not the drug of choice for OCD due to its side effect profile and the availability of more effective and safer options like SSRIs.
**Option B:** Haloperidol - Haloperidol is an antipsychotic medication that is not typically used to treat OCD. It may even exacerbate symptoms of the disorder in some patients.
**Option C:** Lithium - Lithium is a mood stabilizer used primarily to treat bipolar disorder and is not a first-line treatment for OCD.

**Clinical Pearl / High-Yield Fact**
SSRIs like fluoxetine are effective in reducing symptoms of OCD, but they may take several weeks to produce a noticeable effect. Therefore, patients with OCD should be advised to continue treatment for at least 6-12 weeks before considering a change in medication.

**Correct Answer: C. Fluoxetine.**