Here's why:

* Electron affinity is the energy change that occurs when an electron is added to a neutral atom in the gaseous state to form a negative ion.

* Halogens are Group 17 elements (also known as Group VIIA) and have a high tendency to gain an electron to achieve a stable octet configuration.

* As you move down the halogen group, the atomic radius increases. This means the outermost electron is further from the nucleus and experiences less attraction.

* The weaker the attraction between the nucleus and the incoming electron, the less energy is released (i.e., the less negative the electron affinity).

Therefore, iodine, being the largest halogen, has the lowest electron affinity compared to fluorine (F), chlorine (Cl), bromine (Br), and astatine (At).