#### Answer

Arithmetic sequence and the common difference is $ d=1$.

#### Work Step by Step

We are given that $ a_n=n-3$
Here, we have the first four terms $ a_1=-2, a_2=-1,a_3=0,a_4=1$ when $ n=1,2,3,4$
Now, subtract each two consecutive terms as below: $ a_2-a_1=-1+2=1; a_3-a_2=0+1=1; a_4-a_3=1-0=1$
We can see that this is an arithmetic sequence and the common difference is $ d=1$.