SOLUTION :
The amount of milk in the ratio $=\frac{70 \times 3}{5}=42 \mathrm{~kg}$
$\therefore \quad$ The amount of water $=70-42=28 \mathrm{~kg}$
The new amount of water
$=28+14=42 \mathrm{~kg}$
$\therefore \quad$ Required ratio
$=\frac{42}{42}=\frac{1}{1}=1: 1$; Ans.
$\therefore \quad$ Correct option is $(\mathrm{C})$