**Core Concept**
A line diagram is a type of graphical representation used to display changes in data over a continuous interval or period of time. This visual aid is particularly useful for illustrating trends, patterns, and fluctuations in data, making it an ideal choice for depicting incidence changes over a specified period.

**Why the Correct Answer is Right**
A line diagram effectively conveys the direction and magnitude of changes in incidence rates over time. By plotting the data as a continuous line, it becomes easier to identify increases, decreases, or stability in the trend. This is especially useful for tracking changes in disease incidence, such as the rise or fall of epidemic curves. The continuous nature of the line diagram allows for the visualization of subtle changes and patterns that may not be apparent in other types of graphs.

**Why Each Wrong Option is Incorrect**
**Option A:** A pie chart is a circular statistical graphic divided into slices to illustrate numerical proportion. It is not suitable for depicting incidence changes over a period, as it does not account for temporal relationships between data points.

**Option B:** A histogram is a graphical representation of the distribution of numerical data. While it can be used to display changes in data, it is not ideal for illustrating trends over time, as it focuses on the distribution of individual data points rather than their temporal relationships.

**Option C:** A scatter diagram, also known as a scatter plot, is a type of graph that displays the relationship between two variables. While it can be used to show changes in data, it is not the best choice for illustrating incidence changes over a period, as it does not account for the temporal sequence of the data.

**Clinical Pearl / High-Yield Fact**
When using line diagrams to depict incidence changes, it is essential to ensure that the data points are plotted in chronological order to accurately reflect the temporal relationships between the data.

**✓ Correct Answer: D. Line diagram. A line diagram is the best statistical data diagram to depict incidence change over a period of time.**