A guard tube, also known as a drying tube, is commonly used in chemistry laboratories to remove moisture from gases. It contains a desiccant, a material that absorbs water. Among the options provided, the correct material that is commonly used in a guard tube for the preparation of HCl gas is calcium chloride.

Calcium chloride is a hygroscopic substance, meaning it has a strong affinity for water and readily absorbs moisture from the air. When HCl gas is being prepared, it is essential to remove water vapor from the gas stream to obtain pure HCl. Calcium chloride in the guard tube acts as a desiccant and absorbs any moisture present in the gas, ensuring that the gas obtained is dry and free from water vapor.

The other options listed - calcium bromide, calcium iodide, and calcium fluoride - are not typically used in guard tubes for drying HCl gas. Although calcium bromide, calcium iodide, and calcium fluoride also have some drying capabilities, calcium chloride is the most commonly used desiccant for this purpose due to its high efficiency in removing water vapor.