(N/A) The dipole moment of $NH_{3}$ is higher than that of $NF_{3}$.
Dipole moment of $NH_{3} = 1.47 \ D = 4.9 \times 10^{-30} \ Cm$
Dipole moment of $NF_{3} = 0.23 \ D = 0.8 \times 10^{-30} \ Cm$
$\mu(NH_{3}) > \mu(NF_{3})$: Both molecules have a pyramidal shape with a lone pair of electrons on the nitrogen atom.
Electronegativity values are: $H(2.1)$,$N(3.0)$,and $F(4.0)$. Nitrogen is partially negative in $NH_{3}$ but partially positive in $NF_{3}$ due to the high electronegativity of fluorine.
In $NH_{3}$,the orbital dipole due to the lone pair is in the same direction as the resultant dipole moment of the $N-H$ bonds,which adds up to a larger total dipole moment.
In $NF_{3}$,the orbital dipole due to the lone pair is in the direction opposite to the resultant dipole moment of the three $N-F$ bonds. This opposes the bond moments,resulting in a significantly lower net dipole moment.