If a molecule MX3 has zero dipole moment the sigma bonding orbitals used by M (atomic number < 21) are

Dipole moment of MX3 is zero.It indicates MX3 is a symmetrical molecule.In general (not always) molecules with regular geometry (molecules not containing lone pairs on central atom) are symmetrical.Thus MX3 must be AB3 type with 3 bond pairs and no lone pairs on cental atom. Hybridization on "M" in MX3 will be sp2Ex:-