Step 1: Reduce to a comparison
Since $i$ and $K_f$ are the same for all four urea solutions, ranking the depressions is the same as ranking the molalities.
Step 2: Pick the smallest
The lowest molality is 0.06 m, so it gives the least lowering of the freezing point. Answer (D).
Final Answer:
The least concentrated solution, 0.06 m, has the minimum freezing point depression, option (D).
\[ \boxed{0.06\ \text{m}} \]