Answer:
A. [tex]\text{r} = \frac{\text{E}-\text{v}}{\text{I}}[/tex]
Step-by-step explanation:
E = v + Ir
E - v = Ir (subtract by v on both sides)
(E-v)/I = r (divide by I on both sides)
[tex]\therefore \text{r} = \frac{\text{E}-\text{v}}{\text{I}}[/tex]