$\dfrac{L_c}{I} = \dfrac{20}{D}$

$\dfrac{L_{c1}}{24^\circ} = \dfrac{20}{6^\circ}$

$L_{c1} = 80 ~ \text{m}$

$\begin{align}

\text{Sta.} ~ P.C.C & = \text{Sta.} ~ P.C. + \, L_{c1} \\

& = (10 + 420) + 80 \\

& = \text{km} ~ 10 + 500

\end{align}$

Answer: [ D ]