I need help with this question involving an A.P.

The question says: If the sum of terms of an A.P is then the sum of squares of these terms will be equal to:

(a) (b) (c) (d)

What I did: Let the sum of n terms of the A.P be .

...(i)

When n=1

In equation (i)

Using (ii)

I have found the value of . What should I do next?