**Core Concept**
Hallucinations are a type of perceptual disturbance characterized by the experience of sensory stimuli without an external stimulus. They can occur in any sensory modality, including visual, auditory, olfactory, gustatory, or tactile. Hallucinations are often associated with various psychiatric and neurological conditions.

**Why the Correct Answer is Right**
The correct answer, **D. Always pathological**, is incorrect because not all hallucinations are necessarily pathological. While hallucinations are commonly associated with psychiatric disorders such as schizophrenia, they can also occur in normal individuals, particularly in situations where the senses are overstimulated or fatigued, such as during sensory deprivation or in the context of certain sleep disorders. Additionally, some hallucinations may be a normal part of the aging process, such as the hallucinations experienced by some older adults with dementia.

**Why Each Wrong Option is Incorrect**
**Option A:** Hallucinations can indeed occur in any sensory modality, making this statement true. Hallucinations can affect any of the senses, including vision, hearing, smell, taste, or touch.
**Option B:** Hallucinations are indeed independent of the observer's will, meaning that the individual experiencing the hallucination cannot control or wish it away. This is a fundamental characteristic of hallucinations.
**Option C:** This statement is also true, as hallucinations are defined as perceptions that occur without an external stimulus.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that not all hallucinations are indicative of a underlying psychiatric or neurological disorder. A thorough clinical evaluation is necessary to determine the cause and significance of hallucinations.

**✓ Correct Answer: D. Always pathological**