The correct option is A [(g1)n1.(g2)n2]1n1+n2
Let x1,x2,....xn1 and y1,y2,....yn2 be two series of sizes n1 and n2 respectively.
Now, g1=(x1x2⋯xn1)1n1
⇒gn1=x1x2....xn ....(1)
g2=(y2y2⋯yn2)1n2
⇒gn2=y1y2....yn .....(2)
Total number of terms in combined series =n1+n2
If g is the G.M.of the combined series g={(x1x2⋯xn1)×(y1y2⋯yn2)}1n1+n2
g={gn11.gn22}1n1+n2