Step 1: Concept
If $k$ geometric means are inserted between $l$ and $n$, the total number of terms in the G.P. is $k + 2$.
Step 2: Analysis
Here, $k = 3$ ($G_1, G_2, G_3$), so the total terms are 5.
The 1st term $a = l$ and the 5th term $a_5 = n$.
Step 3: Calculation
$a_5 = ar^4 \implies n = l \cdot r^4$.
$r^4 = \frac{n}{l} \implies r = (\frac{n}{l})^{1/4}$.
Final Answer: (D)