a
Both the molecules have a pyramidal shape with a lone pair of electrons on the nitrogen atom. Although fluorine is more electronegative than nitrogen, the resultant dipole moment of \(\mathrm{NH}_{3}\left(4.90 \times 10^{-30} \mathrm{CM}\right)\) is greater than that of \(\mathrm{NF}_{3}\left(0.8 \times 10^{-30} \mathrm{Cm}\right)\)

This is because in case of \(\mathrm{NH}_{3}\) the orbital dipole due to lone pair is in the same direction as the

resultant dipole moment of the \(\mathrm{N}-\mathrm{H}\) bonds. Whereas in \(\mathrm{NF}_{3}\) the orbital dipole is in the direction opposite to the resultant dipole moment of

the three \(N\) - F bonds. The orbital dipole because of the lone pair decreases the effect of the

resultant \(N\) - F bond moments, which results in the low dipole moment of \(\mathrm{NF}_{3}\).