The correct option is C 5Ω should be connected in series with B
a) here, A=10||10=10/2=5,B=5,C=4,D=4
and A/B=1,C/D=1 so, A/B=C/D
b) here, A=10+10=20,B=5,C=4,D=4
and A/B=4,C/D=1 so,A/B≠C/D
c) here,A=10,B=5+5=10,C=4,D=4
and A/B=1,C/D=1 so, A/B=C/D
d)here, A=10,B=5||5=5/2=2.5,C=4,D=4
And A/B=4,C/D=1 so,A/B≠C/D