[tex] \dfrac{9mn-6m}{25-m^2}\div\dfrac{12mn}{5+m}=\dfrac{3m(3n-2)}{5^2-m^2}\cdot\dfrac{5+m}{3m\cdot4n}\\\\=\dfrac{3n-2}{(5-m)(5+m)}\cdot\dfrac{5+m}{4n}=\dfrac{3n-2}{4n(5-m)}\to\boxed{C.} [/tex]
