Explanation

(d) Acceleration due to gravity at depth $d$ below the surface earth
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ g_{d} = g\left( 1 - \frac{d}{R} \right)$
Acceleration due to gravity at height $h$ from the surface of the earth
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ g_{h} = g\left( g - \frac{2h}{R} \right)$
$Given\ \ \ \ \ \ \ \ \ \ \ \ \ \ g_{h} = g_{d}$
$\therefore\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \frac{2h}{R} = \frac{d}{R}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ d = 2h$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ d = 10\ km\ $