If the diameter of a circle reduced by 50%, then the area of the circle reduced by

The area of a circle is $\frac{\pi d^2}{4}$, so it varies with the square of the diameter. Halving the diameter makes the area $\left(\frac12\right)^2=\frac14$ of the original, a reduction of $\frac34$, which is 75%. 50% would be right only if area varied directly with the diameter.