**Core Concept**
'Snowball' opacities refer to a specific type of retinal change seen in patients with a particular condition. These opacities are characterized by rounded, fluffy lesions near the ora serrata, which is the serrated edge of the retina where it meets the vitreous humor.

**Why the Correct Answer is Right**
The correct answer is associated with Coat's disease, a rare eye disorder that primarily affects children. Coat's disease is characterized by abnormal development of blood vessels in the retina, leading to retinal detachment and vision loss. The 'snowball' opacities in Coat's disease are due to the abnormal blood vessels, which leak fluid and cause retinal edema. The ora serrata is a common site for these opacities because it is a region of high metabolic activity and prone to abnormal vascular development.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because 'snowball' opacities are not typically associated with retinoblastoma, a malignant tumor of the retina that primarily affects children. While retinoblastoma can cause retinal changes, it is not characterized by 'snowball' opacities.

**Option B:** This option is incorrect because 'snowball' opacities are not typically associated with diabetic retinopathy, a complication of diabetes that causes damage to the blood vessels in the retina. While diabetic retinopathy can cause retinal changes, it is not characterized by 'snowball' opacities.

**Option C:** This option is incorrect because 'snowball' opacities are not typically associated with retinal detachment, a condition where the retina separates from the underlying tissue. While retinal detachment can cause retinal changes, it is not characterized by 'snowball' opacities.

**Clinical Pearl / High-Yield Fact**
Coat's disease is often bilateral and can be associated with systemic conditions such as hypertension and atherosclerosis. It is essential for ophthalmologists to recognize the characteristic 'snowball' opacities near the ora serrata as a sign of Coat's disease, which requires prompt treatment to prevent vision loss.

**Correct Answer:** C. Coat's disease