The pair from the following pairs having both compounds with net non-zero dipole moment is

Solution

Chemistry ma’am · 3 steps

Ask of each molecule: do the bond dipoles cancel by symmetry? One zero kills the pair.

1. The test. A molecule has zero dipole moment only when its polar bonds point in directions that cancel exactly. That needs a symmetric shape, such as a centre of symmetry or identical groups arranged evenly.
2. Chloromethanes. In CH₂Cl₂ and CHCl₃ the four groups on carbon are not all the same, so the tetrahedral bond dipoles cannot cancel. Only CH₄ and CCl₄, with four identical groups, come out at zero.

   Option	First compound	Second compound	Both non-zero?
   (a)	cis-2-butene: both CH₃ on one side, non-zero	trans-2-butene: CH₃ groups opposite, zero	No
   (b)	CH₂Cl₂: unsymmetric tetrahedron, non-zero	CHCl₃: one H breaks symmetry, non-zero	Yes
   (c)	Benzene: non-polar, zero	Anisidine (OCH₃ and NH₂ on the ring): non-zero	No
   (d)	1,4-dichlorobenzene: C–Cl dipoles opposite, zero	1,3-dichlorobenzene: dipoles at 120°, non-zero	No

3. So. Every other pair contains one symmetric, zero-dipole molecule. Only (b), CH₂Cl₂ and CHCl₃, has both compounds polar.

The idea to keep. Dipole moment is a vector sum. Look for symmetry: trans isomers, para-disubstituted rings with identical groups and CX₄-type molecules usually cancel to zero.

Answer from the NTA final answer key. Solution by PrepWiser.