Answer
Geometric sequence.
Work Step by Step
In order for a sequence to be geometric, the quotient of all consecutive terms must be constant.
Here: $\frac{a_2}{a_1}=\frac{6}{2}=3$
$\frac{a_3}{a_2}=\frac{18}{6}=3$
$\frac{a_4}{a_3}=\frac{54}{18}=3$
Thus we can see that the quotient is constant, thus it is a geometric sequence.