**Core Concept**
The retina is a complex neural tissue located at the innermost layer of the eye. It is responsible for converting light into electrical signals that are transmitted to the brain, allowing us to perceive vision. The retina is a specialized structure composed of multiple layers of neurons, photoreceptors, and blood vessels.

**Why the Correct Answer is Right**
The correct answer is that the retina is part of the eye. The retina is a critical component of the eye, working in conjunction with the lens and cornea to focus light onto the photoreceptors. The retina's neural tissue is continuous with the optic nerve, which transmits visual information from the eye to the brain. The retina's blood supply comes from the retinal arteries and veins, which branch off from the central retinal artery and vein.

**Why Each Wrong Option is Incorrect**
**Option A:** The retina is not part of the brain, although it is often referred to as the "eye's brain" due to its complex neural structure. The brain is responsible for processing visual information, but the retina is the structure that initially captures and transmits this information.
**Option B:** The retina is not part of the ear, which is responsible for detecting sound waves and transmitting auditory information to the brain.
**Option C:** The retina is not part of the skin, which is the largest organ of the body and provides a barrier against external factors.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the retina is a highly specialized tissue that is vulnerable to damage from conditions such as diabetes, hypertension, and trauma. Retinal damage can lead to vision loss and is a common cause of blindness worldwide.

**Correct Answer:** D. The retina is part of the eye.