The atomic number of the element $\mathrm{M}$ is given as less than $21$. The electronic configuration for an atom with an atomic number of $21$ will be $\left[\mathrm{Ar}\right]3{\mathrm{s}}^{2}3{\mathrm{p}}^1$. This implies that the element $\mathrm{M}$ with less than $21$ electrons does not have $\mathrm{d}$-orbital electrons. 

If the molecule ${\mathrm{MX}}_3$ is non-polar in nature, and contains no $\mathrm{d}$-orbitals, then it must have three sigma (single) bonds. The hybridisation for three sigma bonds is ${\mathrm{sp}}^2$. Also, the fact that the molecule has zero dipole moment confirms that the molecule has a symmetrical geometry of triagonal planar arrangement, which is possible only for an ${\mathrm{sp}}^2$hybridisation around $\mathrm{M}$.