The molecule having smallest bond angle is:

All these molecules are ${\text{MX}}_3$ type with one lone pair (pyramidal geometry).As we go down the group from $\text{N}\to \text{P}\to \text{As}\to \text{Sb}$, the size of the central atom increases and its electronegativity decreases.The bond pairs move further away from the central atom, reducing bond pair-bond pair repulsion. Consequently, the lone pair-bond pair repulsion causes the bond angle to contract more.Order of bond angles: ${\text{NCl}}_3>{\text{PCl}}_3>{\text{AsCl}}_3>{\text{SbCl}}_3$.${\text{SbCl}}_3$ has the smallest bond angle.