Explanation

(c)
Electronegativity difference between N (3,0) and Cl (3.0) is zero and hence, $N - Cl$ bonds are non-polar. As a result, the overall dipole moment of $NCl_{3}$ molecule and its direction is just the dipole moment of the lone pair of electrons
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On the other hand, $N - Br,\ (3.0 - 2.8),N - I(3.0 - 2.5)\ and\ N - H\ (3.0 - 2.1)$ are polar and hence, contribute towards the overall dipole moment of the respective moleculas. Since, the EN difference is higher in case of $N - H$ bonds, therefore, $NH_{3}$has the higher dipole moment