Step 1: Concept
The plane $z = 0$ (the $xy$-plane) divides the line segment joining $(x_1, y_1, z_1)$ and $(x_2, y_2, z_2)$ in the ratio $-z_1 : z_2$.
Step 2: Meaning
For the points $P(2, 4, 5)$ and $Q(3, 5, -4)$, we have $z_1 = 5$ and $z_2 = -4$.
Step 3: Analysis
Using the ratio formula:
\[ \text{Ratio} = -\frac{z_1}{z_2} = -\frac{5}{-4} = \frac{5}{4} = 5 : 4 \]
Since the ratio is positive, the division is internal.
Step 4: Conclusion
Therefore, the $xy$-plane divides the segment in the ratio $5 : 4$ internally.
Final Answer: (A)