**Core Concept**
A frequency polygon is a graphical representation used in statistics to display the distribution of data. It is a line graph that connects the midpoints of the tops of the histogram blocks, providing a visual representation of the underlying data distribution.

**Why the Correct Answer is Right**
The correct answer is D. Histogram blocks because a frequency polygon is obtained by joining the midpoints of the tops of the histogram blocks. Histograms are graphical representations of the distribution of numerical data, and by joining the midpoints of the histogram blocks, we can create a frequency polygon that displays the underlying data distribution. This allows for a more detailed and nuanced understanding of the data, including the shape, central tendency, and variability of the data.

**Why Each Wrong Option is Incorrect**
**Option A:** A bar diagram is a graphical representation that displays the frequency or proportion of different categories, but it is not used to create a frequency polygon.

**Option B:** A complete bar diagram is a type of bar diagram that displays both the frequency and percentage of different categories, but it is not used to create a frequency polygon.

**Option C:** A multiple bar diagram is a graphical representation that displays the frequency or proportion of different categories across multiple groups, but it is not used to create a frequency polygon.

**Clinical Pearl / High-Yield Fact**
When creating a frequency polygon, it is essential to note that the midpoints of the histogram blocks are used to connect the points, rather than the actual data points themselves. This allows for a more accurate representation of the underlying data distribution.

**Correct Answer:**
✓ Correct Answer: D. Histogram blocks