Which of the following has zero dipole moment?
A. BF3

B. BeClBr
C. CH2Cl2

D. COS

To determine which molecule has zero dipole moment, let's consider the molecular geometry and the polarities of the bonds within each molecule.

A. BF3:
BF3 has a trigonal planar geometry with three identical B-F bonds. Since the fluorine atom is more electronegative than boron, each B-F bond is polar, but they cancel each other out due to the symmetric arrangement, resulting in a molecule with zero dipole moment.

B. BeClBr:
BeClBr also has a trigonal planar geometry. However, unlike BF3, BeClBr has different atoms (chlorine and bromine) bonded to the central beryllium atom. The polarities of the Cl-Be and Br-Be bonds do not cancel each other out due to the different electronegativities of chlorine and bromine, resulting in a non-zero dipole moment.

C. CH2Cl2:
CH2Cl2 has a tetrahedral geometry with four different atoms bonded to the central carbon atom. The C-H bonds are non-polar, but the C-Cl bonds are polar due to the difference in electronegativity between carbon and chlorine. The dipole moments of the C-Cl bonds do not cancel out due to the asymmetrical arrangement, resulting in a non-zero dipole moment for the molecule.

D. COS:
COS has a linear geometry with a carbon-oxygen double bond and a sulfur atom bonded to the carbon atom. The C=O bond is polar, but the C=S bond is less polar. However, the opposing directions of the bond dipoles cancel each other out, resulting in a molecule with zero dipole moment.

So, the correct answer is:
D. COS