The correct answer is $\frac{G}{n}$.
An ammeter is a device used to measure current. It is connected in series with the circuit being measured, and the current flowing through the ammeter is proportional to the voltage drop across the ammeter. The resistance of an ammeter is very low, so that it does not significantly affect the current flowing through the circuit.
To convert an ammeter with a range of $I$ amperes to an ammeter with a range of $nI$ amperes, a resistance of $\frac{G}{n}$ is connected in parallel with the ammeter. This increases the total resistance of the ammeter to $G + \frac{G}{n} = \frac{nG}{n} = G$, so that the current flowing through the ammeter is still proportional to the voltage drop across the ammeter. However, the voltage drop across the ammeter is now $nI$ times larger, so the current flowing through the ammeter is also $nI$ times larger.
Option A is incorrect because it would result in an ammeter with a resistance of $nG$, which is too high. Option B is incorrect because it would result in an ammeter with a resistance of $(n-1)G$, which is too low. Option C is incorrect because it would result in an ammeter with a resistance of $\frac{G}{n-1}$, which is too high. Option D is correct because it would result in an ammeter with a resistance of $\frac{G}{n}$, which is the correct value to convert the ammeter from a range of $I$ amperes to a range of $nI$ amperes.