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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.