**Core Concept**
The patient's symptoms suggest a primary obsessive-compulsive disorder (OCD), characterized by recurrent, intrusive thoughts (obsessions) and repetitive behaviors (compulsions) aimed at reducing anxiety. In this case, excessive handwashing is a compulsive behavior driven by obsessive thoughts about contamination.

**Why the Correct Answer is Right**
The best mode of treatment for OCD is a combination of pharmacotherapy with a selective serotonin reuptake inhibitor (SSRI) and cognitive-behavioral therapy (CBT), specifically exposure and response prevention (ERP). SSRIs, such as fluoxetine or sertraline, work by increasing serotonin levels in the brain, which helps reduce the frequency and severity of obsessions and compulsions. ERP, a type of CBT, involves helping the patient gradually confront situations that trigger their obsessions while preventing them from engaging in their compulsive behaviors.

**Why Each Wrong Option is Incorrect**
**Option A:** Medications like benzodiazepines or antipsychotics may provide temporary relief but are not the primary treatment for OCD and can have significant side effects.
**Option B:** Relaxation techniques, such as deep breathing or progressive muscle relaxation, may help manage anxiety but do not address the underlying obsessions and compulsions of OCD.
**Option C:** Behavioral therapy alone, without medication, may not be sufficient to manage severe OCD symptoms.
**Option D:** Hospitalization is usually not necessary for OCD treatment, unless the patient is a danger to themselves or others due to their symptoms.

**Clinical Pearl / High-Yield Fact**
SSRIs like fluoxetine can take 4-6 weeks to start showing their therapeutic effects, so it's essential to monitor patients closely during this period and adjust their treatment plan as needed.

**Correct Answer:** B.