Answer: Option B
$$\eqalign{
& {\text{Let}}\,{\text{original}}\,{\text{length}} = x\,{\text{and}}\, \cr
& {\text{Original}}\,{\text{breadth}} = y \cr
& {\text{Original}}\,{\text{area}} = xy \cr
& {\text{New}}\,{\text{length}} = \frac{x}{2} \cr
& {\text{New}}\,{\text{breadth}} = 3y \cr
& {\text{New}}\,{\text{area}} = \left( {\frac{x}{2} \times 3y} \right) = \frac{3}{2}xy \cr
& \therefore {\text{Increase}}\,\% = {\frac{1}{2}xy \times \frac{1}{{xy}} \times 100} \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 50\% \cr} $$