The molecule having smallest bond angle is

The bond angle $\mathrm{Cl}-\mathrm{A}-\mathrm{Cl}$(where $\mathrm{A}$ is$\mathrm{N}, \mathrm{P}, \mathrm{A}$or $\mathrm{Sb}$) decreases with the decrease in the electronegativity or increase in the size of the central atom $\mathrm{A}$. Thus, out of ${\mathrm{NCl}}_3 ,$${\mathrm{PCl}}_3$ , ${\mathrm{AsCl}}_3$ and ${\mathrm{SbCl}}_3$, the latter is expected to have the smallest bond angle.