**Core Concept**
The half-life of a drug refers to the time it takes for the plasma concentration of the drug to decrease by half. It is an essential pharmacokinetic parameter that determines the duration of action of a drug. Different drugs have varying half-lives, which can range from a few minutes to several days.

**Why the Correct Answer is Right**
The longest half-life is typically seen in drugs that are lipophilic and have a large volume of distribution. These drugs are slowly metabolized and eliminated from the body, resulting in a longer half-life. The correct answer is a drug that is known for its long half-life, which allows for once-daily dosing.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it has a shorter half-life, requiring frequent dosing.
**Option B:** This option is incorrect because it has a variable half-life, which can be affected by factors such as liver function and drug interactions.
**Option C:** This option is incorrect because it is primarily excreted by the kidneys, resulting in a shorter half-life.

**Clinical Pearl / High-Yield Fact**
When choosing a medication, consider the half-life of the drug to determine the dosing frequency. Drugs with long half-lives can often be given once daily, while those with short half-lives may require more frequent dosing.

**Correct Answer: D. Olanzapine**