Final answer:
Iodine (I) is the correct answer, as it is the element among the options given that is a halogen, belonging to Group 17 of the periodic table.
Step-by-step explanation:
The question asks which of the following elements is a halogen: Cs (cesium), I (iodine), Ag (silver), or none. Halogens are elements that belong to Group 17 of the periodic table. The halogens include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements are known for being highly reactive nonmetals. They readily react with metals to form various types of salts, such as sodium chloride and calcium chloride, which is why they are called "salt-forming" based on the Greek words from which 'halogen' is derived.
Out of the options provided, iodine (I) is a halogen. Cesium (Cs) belongs to Group 1 and is an alkali metal, and silver (Ag) is a transition metal, not a halogen. Therefore, the correct answer is I (iodine).