A molecule is polar if it has a net dipole moment.

• This depends on:
  1. Presence of polar bonds
  2. Molecular geometry
  3. Whether bond dipoles cancel or not

Check each option:

A: $\mathrm{CCl}_4$

• Geometry: tetrahedral
• Each $\mathrm{C-Cl}$ bond is polar.
• But all four bond dipoles are symmetrically arranged and cancel out.
• Therefore, net dipole moment $=0$.
• Non-polar

B: $\mathrm{CO}_2$

• Geometry: linear
• Each $\mathrm{C=O}$ bond is polar.
• The two bond dipoles are equal and opposite, so they cancel.
• Therefore, net dipole moment $=0$.
• Non-polar

C: $\mathrm{CH_2=CH_2}$

• Ethene is planar and symmetric.
• The bond dipoles cancel due to symmetry.
• Therefore, net dipole moment is zero.
• Non-polar

D: $\mathrm{CHCl_3}$

• Geometry around carbon: tetrahedral
• There are three polar $\mathrm{C-Cl}$ bonds and one $\mathrm{C-H}$ bond.
• Because the substituents are not identical, the dipoles do not cancel completely.
• Hence, the molecule has a net dipole moment.
• Polar

Conclusion: The polar molecule is: $\boxed{\mathrm{CHCl_3}}$

Comparison with stored correct answer:

• Derived answer: $D$
• Stored correct answer: $D$
• They match.