Let the no. of students in front row be x.
So, the no. of students in next rows be x–3,x−6,x–9 ... so on
If n is the no. of rows then no. of students (n=3)
x+(x–3)+(x–6)=630
x=213
Similarly for n=4
x+(x–3)+(x–6)+(x−9)=630
4x–18=630
If n=5
(4x–18)+(x−12)=630
5x–30=630
x=120
Again possible.
If n=6
(5x−30)+(x−15)=630
6x−45=630
6x=675
Hence n ≠ 6
Hence,Option (4)6 is the correct choice.