If a molecule $M X_3$ has zero dipole moment. the sigma bonding orbitals used by M. (atomic number <21) are

$(A)\;Pure\; p \\ (B)\;SP\; hybrid \\(C)\;SP^2\; hybrid \\(D)\;SP^3\; hybrid$

For resultant dipole moment for a molecule $M X_3$ , the molecule must be trigonal i.e $SP^2$ hybridised orbital should be used by M.
Hence C is the correct answer.