If the rectangle a × b and the rectangle c × d are similar, then
a : b = c : d.
Check:
[tex]A)\ \dfrac{6}{8}=\dfrac{6:2}{8:2}=\dfrac{3}{4}\qquad\text{CORRECT}\\\\B)\ \dfrac{6}{8}=\dfrac{6:2}{8:2}=\dfrac{3}{4}\neq\dfrac{2}{4}\\\\C)\ \dfrac{6}{15}=\dfrac{6:3}{15:3}=\dfrac{2}{5}\neq\dfrac{3}{8}\\\\D)\ \dfrac{9}{15}=\dfrac{9:3}{15:3}=\dfrac{3}{5}\neq\dfrac{3}{4}[/tex]