**Core Concept**
Receptor potentials are electrical changes in cells, specifically neurons and muscle cells, that occur in response to the binding of a neurotransmitter or hormone to its corresponding receptor. These changes are essential for initiating and modulating various physiological processes, including sensory perception, muscle contraction, and neurotransmitter release.

**Why the Correct Answer is Right**
Receptor potentials are graded changes, meaning their amplitude depends on the strength of the stimulus. This is in contrast to action potentials, which are all-or-nothing electrical impulses. The graded nature of receptor potentials allows for subtle variations in response to different stimuli, enabling the cell to interpret and process complex information. When a neurotransmitter binds to its receptor, it can either excite (depolarize) or inhibit (hyperpolarize) the cell, leading to a graded change in the membrane potential.

**Why Each Wrong Option is Incorrect**
**Option A:** The difference in permeability of receptors for different ions is not the primary mechanism behind receptor potentials. While ion channels can be activated or inhibited by receptor binding, the graded change in membrane potential is a result of the net movement of ions across the cell membrane, not the receptors themselves.
**Option B:** Receptor potentials are not propagated, meaning they do not travel along the length of the cell like action potentials. Instead, they are localized changes in membrane potential that can modulate cellular behavior.
**Option C:** The refractory period of receptor potentials is not a fixed 15-20 seconds. In fact, receptor potentials can be sustained for as long as the stimulus is present, and their refractory periods can vary depending on the specific receptor and cell type.

**Clinical Pearl / High-Yield Fact**
Receptor potentials are crucial for sensory perception, particularly in the retina and auditory system. For example, photoreceptors in the retina generate graded receptor potentials in response to light, which are then transmitted to the visual cortex for processing. Understanding the graded nature of receptor potentials is essential for appreciating how our senses work.

**✓ Correct Answer: D. Is a graded change**