Sequence and series

Q34 ans is ab
But I am also getting C option as correct
Please can you check if I am right?

@nishantthakre please share your solution

@snehasameera
Please tell my mistake

@nishantthakre
$b^2= ac$ means they are in gp hence numbers in Ap as well as gp. Hemce they are equal.
Hence obviousbly b is right. Since b is right so c is also correct