The dissociation constant of a weak base is ${10}^{-4}$. The concentration ratio of salt to base to be maintained to keep the pH of buffer solution as 9 is

 pH + pOH = 14

9 + pOH = 14

pOH = 14 – 9 = 5

$p{K}_b=-log\left[K_b\right]$ = -log[${10}^{-4}$] = 4

pOH = $p{K}_b+log\frac{\left[salt\right]}{\left[base\right]}$

5 = 4 + $log\frac{\left[salt\right]}{\left[base\right]}$

$log\frac{\left[salt\right]}{\left[base\right]}$ = 5-4

= 1

$\frac{\left[salt\right]}{\left[base\right]}$ = $\frac{{10}^1}{1}$ = $\frac{10}{1}$

Hence, the correct option is option 1) 10: 1.