Q.By long-standing convention in Economics, when drawing a demand-and-supply diagram, which variable is plotted on the X-axis?

The correct option is (D) Pictogram, which is a diagram, not a graph; the other three are all graphs.

In GSEB Class-12 Economics, graphic presentation means plotting a frequency distribution on graph paper. A histogram (adjacent rectangles for class intervals), a frequency polygon (line joining class mid-point heights) and a frequency curve (a smoothed frequency polygon) are all standard graphs. A pictogram shows data using small pictures or symbols (for example, one figure of a person for every lakh of population) and belongs to the family of diagrams, not graphs.

A graph is drawn for a continuous distribution — option (a).

In statistics we present data either through diagrams or through graphs. Diagrams (bar diagrams, pie charts, pictograms) suit discrete/discontinuous data where each category stands separately. A graph, on the other hand, plots values of a continuously varying magnitude (for example a variable measured over time or over a continuous interval) on graph paper and joins the successive points with a line or curve. Joining points is logically valid only when the underlying variable moves smoothly from one value to the next — i.e. when the distribution is continuous.

Hence, for a continuous distribution a graph is the correct tool of presentation, whereas discrete distributions are shown through diagrams.