**Core Concept**
Spring catarrh, also known as allergic conjunctivitis, is a common seasonal eye condition characterized by inflammation and itching of the conjunctiva due to allergen exposure. It typically occurs in response to pollen from trees, grasses, and weeds.

**Why the Correct Answer is Right**
The correct answer, **D. Sodium cromoglycate is a form of therapy**, is true because sodium cromoglycate is a mast cell stabilizer that prevents the release of histamine and other chemical mediators from mast cells, thereby reducing allergic symptoms. It is often used as a first-line treatment for mild to moderate allergic conjunctivitis.

**Why Each Wrong Option is Incorrect**
**Option A:** The cobblestone appearance of the conjunctiva is a hallmark of vernal keratoconjunctivitis (VKC), a more severe and chronic form of allergic conjunctivitis, rather than spring catarrh.

**Option B:** Spring catarrh is indeed common in the spring months, as it is triggered by pollen from trees and grasses that bloom during this time.

**Option C:** Limbus conjunctival thickening is more characteristic of vernal keratoconjunctivitis (VKC), which is a more severe and chronic form of allergic conjunctivitis.

**Clinical Pearl / High-Yield Fact**
To differentiate spring catarrh from vernal keratoconjunctivitis (VKC), look for signs of chronic inflammation, such as limbus conjunctival thickening and cobblestone appearance, which are more typical of VKC.

**✓ Correct Answer: C. Limbus conjunctival thickening**