Correct option is A)
In LR circuit,
The time constant τ=
R
L
10=
R
L
L=10R. . . . . . .(1)
When Resistance 100Ω is connect in series, than the time constant is
τ
′
=
R+100
L
=2s
L=2R+200. . . . . . .(2)
Equating equation (1 ) and (2), we get
2R+200=10R
8R=200
R=25Ω
From equation (1),
L=10R=10×25
L=250H