The correct option is
B GP
Given series
1c,(1ca)12,1a
Lets consider a G.P of elements A,B,C
∴ Geo.mean ⇒B2=AC
Comparing it with given series.
A=1cB=(1ca)12,C=1a
∴B2=(1ca)12×2
=1ca........(1)
AC=1c×1a=1ca..............(ii)
∴(i)=(ii)
∴B2=AC So given series is in G.P