**Core Concept**
The question is testing the concept of allergen exposure prevention in asthmatic patients using pharmacotherapy. Specifically, it involves the administration of a medication that can prevent allergic reactions, such as anaphylaxis or asthma exacerbation, after exposure to an allergen like cat dander.

**Why the Correct Answer is Right**
The correct answer is a leukotriene receptor antagonist (LTRA), such as montelukast. LTRAs work by blocking the action of leukotrienes, which are pro-inflammatory mediators released during an allergic reaction. By preventing the binding of leukotrienes to their receptors, LTRAs can reduce inflammation, smooth muscle contraction, and mucus production in the airways, thereby preventing symptoms of asthma. In this scenario, administering montelukast in the afternoon would have helped prevent the boy's symptoms by blocking the allergic response to cat dander exposure.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not a relevant medication for preventing allergic reactions. **Option B:** This option is not a correct medication for preventing allergic reactions. **Option C:** This option is not a correct medication for preventing allergic reactions.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that LTRAs are effective in preventing allergic reactions when administered before exposure to the allergen. This is why they are often prescribed as a preventive measure for patients with asthma or allergies.

**Correct Answer:** C. Montelukast.