27%

28%

29%

30%

B. 28%

Let original length = x

and original width = y

Decrease in area will be

$=xy-\left(\frac{80x}{100}\times \frac{90y}{100}\right)\phantom{\rule{0ex}{0ex}}=\left(xy-\frac{18}{25}xy\right)\phantom{\rule{0ex}{0ex}}=\frac{7}{25}xy\phantom{\rule{0ex}{0ex}}Decrease=\left(\frac{7xy}{25xy}\times 100\right)\%\phantom{\rule{0ex}{0ex}}=28\%$