**Core Concept:**
Plasma concentration versus time graph is a representation of drug concentration in the blood over time after administration. It provides information about pharmacokinetic parameters like absorption rate, elimination rate, and half-life of the drug.

**Why the Correct Answer is Right:**
The correct answer (D) indicates that the graph shows the concentration of the drug in plasma over time. This helps in understanding the pharmacokinetic parameters of the drug, such as absorption rate (the time it takes for the drug to reach its peak concentration), elimination rate (the rate at which the drug is cleared from the body), and half-life (the time it takes for the drug concentration to decrease to half of its initial value). This information is crucial for adjusting the drug dosage regimen for optimal treatment outcomes.

**Why Each Wrong Option is Incorrect:**
A. This option (C) focuses on the drug concentration at a specific time point rather than the entire concentration-time profile.
B. While drug distribution is a crucial factor in pharmacokinetics, this option (B) does not address the overall concentration-time profile, which is the main focus of the question.
C. This option (A) discusses the drug concentration at different time points, but it does not encompass the entire concentration-time profile, which is the primary information sought in the question.

**Clinical Pearl / High-Yield Fact:**
A well-designed plasma concentration versus time graph helps in determining the optimal drug dosage regimen, ensuring therapeutic drug levels and minimizing side effects. Understanding pharmacokinetic principles, such as those demonstrated by this graph, is essential for safe and effective clinical practice.

**Correct Answer:**
D. Plasma concentration versus time graph