**Core Concept**
The question is testing knowledge of non-sedative antihistamines, which are a class of medications used to treat allergic reactions. These medications work by blocking the action of histamine, a chemical released during an allergic reaction, without causing sedation.

**Why the Correct Answer is Right**
Non-sedative antihistamines, also known as second-generation antihistamines, work by selectively blocking histamine receptors in the brain, which prevents sedation. Examples of non-sedative antihistamines include loratadine, fexofenadine, and desloratadine. These medications are effective in treating allergic symptoms such as itching, sneezing, and runny nose without causing drowsiness.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is a sedative antihistamine, which means it causes drowsiness. First-generation antihistamines, such as diphenhydramine, are known to cause sedation.
* **Option B:** This option is incorrect because it is a sedative antihistamine that is also used to treat insomnia. Second-generation antihistamines are designed to be non-sedative.
* **Option C:** This option is incorrect because it is a sedative antihistamine that is also used to treat motion sickness. First-generation antihistamines, such as dimenhydrinate, are known to cause sedation.
* **Option D:** This option is incorrect because it is a sedative antihistamine that is also used to treat allergic conjunctivitis. First-generation antihistamines, such as pheniramine, are known to cause sedation.

**Clinical Pearl / High-Yield Fact**
When selecting an antihistamine, remember to choose a non-sedative option if you need to remain alert and awake, such as when driving or operating heavy machinery.

**Correct Answer:** D. Pheniramine.