Which answer describe the type of sequence? 3,9,15,21

The sequence is arithmetic

we know that

In an arithmetic sequence, the difference between consecutive terms is always the same and is called common difference

In this problem we have

3,9,15,21, ...

Let

a1=3, a2=9, a3=15, a4=21

a4-a3=21-15=6

a3-a2=15-9=6

a2-a1=9-3=6

The sequence is arithmetic

The common difference is equal to