Which of the following is the lowest ratio ?

Answer: Option A

$$\begin{array}{rl}& \Rightarrow 7:15=\frac{7}{15}=0.466\\ & \Rightarrow 15:23=\frac{15}{23}=0.652\\ & \Rightarrow 17:25=\frac{17}{25}=0.68\\ & \Rightarrow 21:39=\frac{21}{39}=0.538\end{array}$$
Clearly, 7 : 15 is the lowest