The depth d at which the value of acceleration due to gravity becomes $\frac{1}{n}$  times the value at the surface, is [R = radius of the earth]

(a) $\frac{R}{n}$                              (b)  $R\left[\frac{n-1}{n}\right]$

(c) $\frac{R}{{n}^{2}}$                             (d) $R\left[\frac{n}{n+1}\right]$