The overall transfer function O(s)/I(s) is

In order to calculate the overall transfer function of such type of complex block diagram, use direct formula given below. Overall

T. F. =	Sum of total forward path gain
	1 + sum of all loops associated with the gain factors

Here, only one forward path = G1G2G3 Sum of all possible loops = G1G2H1 + G1G2G3H3 + G2G3H2 Hence,

O(s)	=	G₁G₂G₃
I(s)	=	1 + G₁G₂H₁ + G₁G₂G₃H₃ + G₂G₃H₂

Hence alternative (C) is the correct choice.