Probability of getting positive integral roots of the equation $ x^2-n=0$ for the integer $ n \: \: \: 1 \leq n \leq 40$ is

Question

$\begin {array} {1 1} (A)\;\large\frac{1}{5} & \quad (B)\;\large\frac{1}{10} \\ (C)\;\large\frac{3}{20} & \quad (D)\;\large\frac{1}{20} \end {array}$