Find the Radius of Convergence Of $f ^ { \prime } ( x )$

Multiple Choice

Find the radius of convergence of $f ^ { \prime } ( x )$ where $f ( x ) = \sum _ { n = 0 } ^ { \infty } \left( \frac { x } { 6 } \right) ^ { n }$ .

A) 1
B) $\infty$
C) $\frac { 1 } { 6 }$
D) 6
E) 12