When airplane is flying at $Lift/Drag$ ratio of maximum , coefficient of drag at zero lift is equal to coefficient of induced drag.

$Total\, drag = drag\, at\, zero\, lift + induced\, drag$

$${C_D} = {C_{D,0}} + {C_{D,i}}$$

Since $(L/D)$ is here maximum, therefore, $coefficient\, of\, drag\, at\, zero\, lift =coefficient\, of\, induced\, drag$,

$${C_{D,0}} = {C_{D,i}}$$

Therefore,

$${C_D} = 2{C_{D,i}}$$

$${C_{D,i}} = \frac{{C_L^2}}{{\pi eAR}}$$

$$L = \frac{1}{2}{\rho _\infty }V_\infty ^2S{C_L}$$

At, a steady level flight, $L=W$

$$\Rightarrow {C_L} = \frac{{2W}}{{{\rho _\infty }V_\infty ^2S}} = \frac{{2 \times 23000}}{{1.225 \times {{\left( {90} \right)}^2} \times 20}} = 0.232$$

$${C_{D,i}} = \frac{{C_L^2}}{{\pi eAR}} = \frac{{{{\left( {0.232} \right)}^2}}}{{\pi \times 0.9 \times 9}} = 0.0021$$

Therefore,

$${C_D} = 2 \times \left( {0.0021} \right) = 0.0041$$

Total drag,

$$D = \frac{1}{2} \times {\rho _\infty } \times V_\infty ^2 \times S \times {C_{D}}$$

$$\Rightarrow D = \frac{1}{2} \times 1.225 \times {\left( {90} \right)^2} \times 20 \times 0.0041 = 406.823\,N$$