Second term should be 4 right? If i solve it then it comes something as follows-

a1 = 16 (1-r) and a1= 4/r

This implies 16(1-r) = 4/r and the equation thus formed is r^2-16r+4 = 0. Now middle term is not possible here and the answers are given as 2, 6, 8 and 1/2. Then where am I wrong?