**Core Concept**
The treatment of Spring Catarrh, also known as allergic rhinitis, involves the use of medications that reduce inflammation and alleviate symptoms. The condition is characterized by an overactive immune response to allergens, leading to increased mucus production, congestion, and nasal discharge.

**Why the Correct Answer is Right**
The drug of choice for Spring Catarrh is a corticosteroid nasal spray, which works by reducing inflammation and suppressing the immune response. Corticosteroids bind to glucocorticoid receptors in the nasal mucosa, leading to a decrease in the production of pro-inflammatory cytokines and an increase in anti-inflammatory cytokines. This results in reduced swelling, itching, and congestion. The most commonly used corticosteroid nasal spray for Spring Catarrh is Fluticasone propionate.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because antihistamines, such as Diphenhydramine, are not the first-line treatment for Spring Catarrh. While they can provide relief from symptoms like itching and sneezing, they do not address the underlying inflammation.
**Option B:** This option is incorrect because decongestants, such as Oxymetazoline, provide temporary relief from nasal congestion but do not address the underlying inflammation or prevent symptoms from returning.
**Option C:** This option is incorrect because immunotherapy, such as allergy shots, is a long-term treatment option for Spring Catarrh and is not typically used as a first-line treatment.

**Clinical Pearl / High-Yield Fact**
Corticosteroid nasal sprays are effective in reducing symptoms of Spring Catarrh within 12 hours of use, making them a fast-acting treatment option.

**Correct Answer:** C. Fluticasone propionate.