Correct Option: C

In the given figure a.
Here only two forward path and no loop

x4	= abd + acd i.e.,
x1

from figure b Here there are four possible forward path
i.e.,
P₁ = ac, P2 = ad, P3 = bc,
P₄ = bd
Δ₁ = Δ₂ = Δ₃ = Δ₄ = 1
Δ = 1
(∴ There is no individual loop and non-touching loops)
So,

x4	= ab + bd + bc + bd i.e.,
x1

From figure c Here one loop and 2 forward path
P₁ = ac,
P₂ = ad,
D₁ = ab
Δ₁ = Δ₂
Δ = 1 – ab
So,

x4	=	P1Δ1 + P2Δ2
x1		Δ

=	ac·1 + ad·1	=	ac + ad	i.e.,
	1 – ab		1 – ab

from figure d Here one loop and two forward path
P₁ = ac,
P₂ = bc,
D₁ = cd
Δ₁ = Δ₂ = 1
Δ = 1 – cd

x4	=	P1Δ1 + P2Δ2
x1		Δ

So,

x4	=	ac + bc	i.e.,
x1		1 - cd

finally, we get
(a) – 2
(b) – 1
(c) – 4
(d) – 3