Answer
Geometric sequence
Work Step by Step
From the statement, we have $a_n=(\dfrac{1}{2})^{n}$
when $n=1,2,3,4$, then $a_4=${$\dfrac{1}{2},\dfrac{1}{4},\dfrac{1}{8},\dfrac{1}{6}$}. This implies that these terms form an Geometric sequence with common ratio $d=\dfrac{1}{2}$ .
Hence, the correct answer will be Geometric sequence.