Classification – Odd one out (charts/diagrams vs mathematical constant): Pick the item that is unlike the others: Frequency polygon, Rectangle, Bar, Pi.

Introduction / Context:
Maths/graphics classification can hinge on whether an item is a diagrammatic object or a numerical constant. Three options refer to shapes or chart elements commonly used in data display, while one is a fundamental mathematical constant, not a diagrammatic entity.

Given Data / Assumptions:

• Frequency polygon: a chart/graph connecting class midpoints with line segments.
• Bar: a graphical element used in bar charts.
• Rectangle: a geometric shape; also the shape used by bars in charts.
• Pi (π): mathematical constant (~3.14159), ratio of circumference to diameter.

Concept / Approach:
Group “diagrammatic/shape” items (frequency polygon, bar, rectangle) against “constant/number” (pi). The constant is the non-graphical outlier.

Step-by-Step Solution:
Frequency polygon → chart/diagram.Bar → chart element/shape.Rectangle → geometric shape, used in charts.Pi → mathematical constant, not a chart or figure by itself.

Verification / Alternative check:
Even if “rectangle” is pure geometry and the other two are in statistics, all three are diagrammatic/shape concepts; π is purely numeric.

Why Other Options Are Wrong:

• Frequency polygon, Bar, and Rectangle all pertain to visual representations or shapes; π does not.

Common Pitfalls:
Assuming “bar” is ambiguous (metal bar). In a list with frequency polygon and rectangle, the chart/shape sense is clearly intended.

Final Answer:
Pi