Answer
Arithmetic sequence.
Work Step by Step
In order for a sequence to be arithmetic, the difference of all consecutive terms must be constant.
Here: $a_2-a_1=7-2=5$
$a_3-a_2=12-7=5$
$a_4-a_3=17-12=5$
Thus we can see that the difference is constant, thus it is an arithmetic sequence.