We have indeterminateness of type
0/0,
i.e. limit for the numerator is
$$\lim_{x \to 0^+} \tan{\left(m x \right)} = 0$$
and limit for the denominator is
$$\lim_{x \to 0^+} \sin{\left(n x \right)} = 0$$
Let's take derivatives of the numerator and denominator until we eliminate indeterninateness.
$$\lim_{x \to 0^+}\left(\frac{\tan{\left(m x \right)}}{\sin{\left(n x \right)}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{\partial}{\partial x} \tan{\left(m x \right)}}{\frac{\partial}{\partial x} \sin{\left(n x \right)}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{m \left(\tan^{2}{\left(m x \right)} + 1\right)}{n \cos{\left(n x \right)}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{m}{n}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{m}{n}\right)$$
=
$$\frac{m}{n}$$
It can be seen that we have applied Lopital's rule (we have taken derivatives with respect to the numerator and denominator) 1 time(s)