Concept:
For $ax^2 + 2hxy + by^2 = 0$, slopes satisfy:
\[
bm^2 + 2hm + a = 0
\]
Step 1: Use roots relation.
\[
m_1 + m_2 = -\frac{2h}{b}, \quad m_1 m_2 = \frac{a}{b}
\]
Step 2: Apply identity.
\[
(m_1 - m_2)^2 = (m_1 + m_2)^2 - 4m_1m_2
\]
\[
= \frac{4h^2}{b^2} - \frac{4a}{b}
\]
Step 3: Substitute given.
\[
16h^2 = 25ab \Rightarrow h^2 = \frac{25ab}{16}
\]
\[
(m_1 - m_2)^2 = \frac{4}{b^2}\cdot\frac{25ab}{16} - \frac{4a}{b}
\]
\[
= \frac{25a}{4b} - \frac{4a}{b}
= \frac{9a}{4b}
\]
\[
= \frac{9}{4} m_1 m_2
\]
Step 4: Final result.
\[
|m_1 - m_2| = 2
\]
Conclusion: Option (C)