Answer : Option D

Explanation :

Assume that the person would have covered x km if travelled at 10 km/hr

$MF#%\Rightarrow \text{Speed = }\dfrac{\text{Distance}}{\text{Time}} = \dfrac{x}{10}\text{.....(Equation1)}$MF#%

$MF#%\Rightarrow \text{Speed = }\dfrac{\text{Distance}}{\text{Time}} = \dfrac{(x+20)}{14}\text{.....(Equation2)}$MF#%

$MF#%\begin{align}
&\dfrac{x}{10} = \dfrac{(x+20)}{14}\\\\
&14x = 10x + 200\\\\
&4x = 200\\\\
&x = \dfrac{200}{4} = 50
\end{align} $MF#%