Find All Positive Values Of $u$ For Which the Series $\sum _ { m = 1 } ^ { \infty } 6 u ^ { m } 7 m$

Multiple Choice

Find all positive values of $u$ for which the series $\sum _ { m = 1 } ^ { \infty } 6 u ^ { m } 7 m$ converges.

A) $u > 7$
B) $6 < u < \frac { 7 } { e }$
C) $0 < u < \frac { 1 } { e }$
D) $u < 6$
E) $u > \ln 7$