A molecule ${\mathrm{MX}}_3$ has zero dipole moment. The 's' character in the hybridized orbitals of M is

The percentage of s-character in ${\mathrm{sp}}^3, {\mathrm{sp}}^2$ and sp hybridization is $25\%, 33.33\%$, and $50\%$ respectively. 

For non-polar ${\mathrm{MX}}_3$, it must have a triangular planar arrangement, i.e. there should be ${\mathrm{sp}}^2$-hybridization around M. There are two p-orbitals and an s-orbital involved. 

So, s-character is $33.33\%$. 

So, option A is correct.