Iodine (I) is the largest halogen atom.

Explanation:

1. The Halogens:
The halogens are a group of elements in the periodic table known as Group 17 or Group VIIA. They include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements are highly reactive nonmetals and are known for their ability to form halide ions by gaining one electron.

2. Atomic Radius:
The atomic radius is a measure of the size of an atom. It is defined as the distance between the nucleus of an atom and its outermost electron. As we move down a group in the periodic table, the atomic radius generally increases. This is because each successive element has an additional electron shell, which increases the distance between the nucleus and the outermost electrons.

3. Comparison of Atomic Radii:
To determine which halogen atom is the largest, we need to compare their atomic radii.

- Fluorine (F) has the smallest atomic radius among the halogens due to its high effective nuclear charge.
- Chlorine (Cl) is larger than fluorine because it has an additional electron shell.
- Bromine (Br) is larger than chlorine because it has an additional electron shell.
- Iodine (I) is the largest halogen atom because it has an additional electron shell compared to bromine.

Therefore, the correct answer is option 'D', iodine (I), as it has the largest atomic radius among the halogens.

Summary:
Iodine (I) is the largest halogen atom due to its additional electron shell compared to fluorine, chlorine, and bromine. The atomic radius generally increases as we move down the halogen group in the periodic table.