Answer
The geometric sequence and the common ratio is $ r=2$.
Work Step by Step
We are given that $ a_n=2^n $
Here, we have the first four terms $ a_1=2, a_2=4,a_3=8,a_4=16$ when $ n=1,2,3,4$
Now, subtract each two consecutive terms as below: $ a_2/a_1=2; a_3/a_2=2=1; a_4/a_3=\dfrac{16}{8}=2$
We can see that this is the geometric sequence and the common ratio is $ r=2$.
