Q.The construction of demand line or supply line is the result of using

A demand line or supply line is constructed using algebra (linear equations) — the correct option is "Algebra".

A straight-line demand curve or supply curve is simply the graph of a linear equation, for example a demand line Qd = a - bP or a supply line Qs = c + dP, where a, b, c and d are constants. Plotting such a line from its equation — finding intercepts and slope and joining the points — is a task of algebra (the study of linear equations), not of matrices or calculus.

• Matrices are used to solve systems of many equations at once, not to draw a single line.
• Calculus deals with rates of change (marginal concepts, slopes of curves), not the basic construction of a straight line from a linear equation. …

The correct option is (d) Straight line: (y − y1) = m(x − x1) is the point-slope equation of a straight line.

Any first-degree relation between two variables plots as a straight line. The form (y − y1) = m(x − x1) is the point-slope form: it describes the unique straight line that passes through the point (x1, y1) and has slope m. Here m measures the steepness (rise ÷ run) of the line.

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