The dipole moment ($\mu$) of a molecule is a measure of the polarity of its chemical bonds and is a vector quantity. For a molecule to have a significant net dipole moment, it must have polar bonds and an asymmetrical geometry such that the individual bond dipoles do not cancel each other out.

• Molecule (i) - Carbon tetrachloride (CCl\u2084): CCl\u2084 has a tetrahedral geometry. Although the C-Cl bonds are polar, the symmetrical arrangement of four identical C-Cl bonds around the central carbon atom results in the cancellation of individual bond dipoles. Therefore, CCl\u2084 has a net dipole moment of zero ($\mu =0$).

• Molecule (ii) - Carbon dioxide (CO\u2082): CO\u2082 has a linear geometry. The C=O bonds are polar, but due to the symmetrical linear arrangement, the two bond dipoles are equal in magnitude and opposite in direction, canceling each other out. Therefore, CO\u2082 has a net dipole moment of zero ($\mu =0$).

• Molecule (iii) - Water (H\u2082O): H\u2082O has a bent (V-shaped) geometry due to the presence of two lone pairs on the central oxygen atom. The O-H bonds are polar, and because of the bent geometry, the individual bond dipoles do not cancel out. Instead, they add up to give a significant net dipole moment ($\mu \ne 0$).

• Molecule (iv) - Ammonia (NH\u2083): NH\u2083 has a trigonal pyramidal geometry due to the presence of one lone pair on the central nitrogen atom. The N-H bonds are polar, and the lone pair also contributes to the overall dipole moment. Due to the asymmetrical pyramidal geometry, the individual bond dipoles and the lone pair dipole do not cancel out, resulting in a significant net dipole moment ($\mu \ne 0$).

Therefore, molecules (iii) and (iv) have a significant dipole moment ($\mu \ne 0$).

Option Analysis:

• A) Only (i): Incorrect, CCl\u2084 has $\mu =0$.

• B) (i) and (ii): Incorrect, both CCl\u2084 and CO\u2082 have $\mu =0$.

• C) Only (iii): Incorrect, NH\u2083 also has $\mu \ne 0$.

• D) (iii) and (iv): Correct, both H\u2082O and NH\u2083 have $\mu \ne 0$.

Correct Answer: (D)