Answer
The sequence is arithmetic.
Common difference $d=4$.
Work Step by Step
We compute the difference between each term and the preceding term. If the difference is same, then the sequence is arithmetic.
$a_{2}-a_{1}=16-12=4$
$a_{3}-a_{2}=20-16=4$
$a_{4}-a_{3}=24-20=4$
$a_{5}-a_{4}=28-24=4$
$a_{6}-a_{5}=32-28=4$
The common difference $d=4$ is constant and therefore the sequence is arithmetic.