The correct option is (C) [η / (1 + η)].
Explanation:
Let internal resistance is r and external resistance in R.
∴ R = ηr
we know V = IR and E = I(R + r)
Where V is potential across source and E is the emf
∴ (V/E) = [(IR) / {I(R + r)}]
= [R / (R + r)]
= [ηr / (ηr + r)]
= [η / (1 + η)]