Solution:
The reluctancy of s-electrons to participate in bonding is the inert pair effect. This effect is more pronounced in heavier metals where s-electrons do not show participation in bonding. This effect is observed on moving down in a group.
There are two oxidation states of group-13 elements: +3 and +1. The stability of the +1 oxidation state increases down the group because of the inert pair effect. The oxidation state of group-13 element in monohalide is +1. This implies that the stability of monohalides increases on moving down the group.