**Core Concept**
The patient's presentation of waxy flexibility, negativism, and rigidity suggests a condition characterized by a decrease in dopamine and an increase in acetylcholine activity in the basal ganglia. This is a classic symptom complex associated with a specific type of psychiatric disorder.

**Why the Correct Answer is Right**
The most probable diagnosis is Catatonia, a neuropsychiatric disorder that typically presents with immobility, mutism, and rigidity. The pathophysiology of catatonia is thought to involve an imbalance between dopamine and acetylcholine activity in the basal ganglia, leading to an inhibition of motor activity. This is often seen in patients with schizophrenia, but it can also occur as a separate entity or in association with other medical conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while schizophrenia is a psychiatric disorder that can present with catatonic symptoms, it is not the most probable diagnosis based on the patient's presentation of waxy flexibility and rigidity.
**Option B:** This option is incorrect because depression is a mood disorder that typically presents with symptoms of sadness, anhedonia, and changes in appetite or sleep, rather than catatonic symptoms.
**Option C:** This option is incorrect because bipolar disorder is a mood disorder that typically presents with symptoms of mania or depression, rather than catatonic symptoms.

**Clinical Pearl / High-Yield Fact**
Catatonia is a neuropsychiatric disorder that can be treated with benzodiazepines or electroconvulsive therapy (ECT), which can rapidly improve symptoms. It is essential to recognize catatonia early and initiate treatment promptly to prevent complications.

**Correct Answer:** D. Catatonia