**Core Concept**
The patient's symptoms, including a fixed, false belief that others are conspiring against him, and a preoccupation with being followed or watched, suggest a thought disorder characterized by a disconnection from reality.

**Why the Correct Answer is Right**
This patient's symptoms are indicative of a paranoid delusion, which is a type of delusion characterized by a fixed, false belief that others are conspiring against him. The patient's preoccupation with being followed or watched, and his denial of auditory hallucinations, suggest that this delusion is not accompanied by a loss of contact with reality in other areas. The absence of other symptoms such as disorganized thinking, disorganized behavior, or negative symptoms, suggests that the patient's thought disorder is primarily delusional in nature.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the patient's symptoms do not meet the criteria for schizophrenia, which typically requires a combination of delusions, hallucinations, disorganized thinking, and disorganized behavior.

**Option B:** This option is incorrect because the patient's symptoms do not meet the criteria for a mood disorder, which typically involves a disturbance of mood such as depression or mania, and is not primarily characterized by delusions.

**Option C:** This option is incorrect because the patient's symptoms do not meet the criteria for a dissociative disorder, which typically involves a disconnection from one's thoughts, feelings, or experiences, and is not primarily characterized by delusions.

**Option D:** This option is incorrect because the patient's symptoms do not meet the criteria for a personality disorder, which typically involves a long-standing pattern of maladaptive behavior and is not primarily characterized by delusions.

**Clinical Pearl / High-Yield Fact**
Paranoid delusions can be a feature of several psychiatric disorders, including schizophrenia, bipolar disorder, and delusional disorder. However, in the absence of other symptoms, a diagnosis of delusional disorder should be considered.

**Correct Answer:** C. Delusional Disorder