Given the sequence 3 , 6 , 9 , 12 , ......
So, the difference between the first two terms = 6 - 3 = 3
the difference between term 6 and 9 = 9 - 6 = 3
So, there is a constant common difference between the terms
which equal to 3
so, the given sequence is Arithmetic
And the next term = 12 + 3 = 15
So, the answer is a. Arithmetic, 15