(a) Calcium chloride
Calcium chloride is used as a drying agent to absorb moisture and prepare dry HCl gas, as it does not react with HCl.
The other options (CaO, Ca(OH)₂, CaCO₃) are basic substances that would react with acidic HCl gas, not just dry it. Only anhydrous CaCl₂ acts as a neutral drying agent suitable for HCl. Note: The source passages do not cover this topic; the answer is based on standard Class 10 Chemistry knowledge of drying agents.