Complete the number series: 3, 7, 6, 5, 9, 3,12, 1,15, ……

A) 21

B) -3

C) -1

D) 13

There are two series, beginning respectively with 3 and 7. In one 3 is added and in another 2 is subtracted.
The next number is 1 – 2 = -1

1st Series          2nd Series 
3 + 3 =6              7 – 2 = 5
6 + 3 = 9             5 – 2 = 3
9 + 3 = 12           3 – 2 = 1
12 + 3 = 15         1 -2  = -1