Properties of Triangles — MATHS STD 7 — Question

Suppose $BC$ is the ladder which is placed againts the wall $OA.$
The foot of the ladder $C$ is $15m$ away from the foot $O$ of the wall and its top reaches the window which is $20m$ above the ground.
In right traingle $A B C$
$\mathrm{AB}^2=\mathrm{BC}^2+\mathrm{AC}^2$
$\Rightarrow \mathrm{AB}^2=(5)^2+(5)^2$
$\Rightarrow \mathrm{AB}^2=25+25$
$\Rightarrow \mathrm{AB}^2=50$
$\Rightarrow \mathrm{AB}^2=(5 \sqrt{2})^2$
$\Rightarrow \mathrm{AB}=5 \sqrt{2} \mathrm{~cm}$
Hence, the correct answer is option $(b).$