Step-by-step explanation:
Arsenic is a group 15 element and lies in period 4. Whereas elements of halogen belongs to group 17.
Elements of group 17 are F, Cl, Br, I. It is known that on moving down the group there will be increase in size of atom because of increase in number of electrons.
Therefore, we can conclude that iodine is the halogen which has a larger average atomic mass than arsenic.