Answer
$128$
Work Step by Step
$a_1=1$ and we can see that the ratio of subsequent terms is $2$, thus $r=2$
The nth term of a geometric series can be obtained by the following formula: $a_n=a_1\cdot r^{n-1}$, where $a_1$ is the first term and $r$ is the common ratio.
Hence here: $a_8=1\cdot2^{8-1}=1\cdot128=128$