Answer
$r=-\frac{1}{2}$
Work Step by Step
The common ratio for the geometric sequence:
$\frac{1}{3},-\frac{1}{6}, \frac{1}{12}, -\frac{1}{24}$
is
$r=\frac{a_{n+1}}{a_n}$
$r=\frac{a_2}{a_1}=\frac{-\frac{1}{6}}{\frac{1}{3}}=-\frac{1}{2}$