Step 1: Understanding the Concept:
Several common acids carry old apothecary names from before modern chemical naming was standardized. The question wants the acid known as "Spirit of Salt."
Step 2: Key Fact:
Hydrochloric acid was traditionally made by heating sodium chloride, ordinary table salt, with an acid such as sulfuric acid, releasing hydrogen chloride gas that was dissolved in water. Because salt was the starting material, early chemists called the product "Spirit of Salt."
Step 3: Detailed Explanation:
Nitric acid is historically known as "aqua fortis," not "Spirit of Salt."
Boric acid comes from natural borate minerals and carries no such traditional name.
Thioglycolic acid is a modern organosulfur compound with no historical link to salt.
Only hydrochloric acid's traditional preparation from salt matches the name in the question.
Step 4: Final Answer:
"Spirit of Salt" refers to hydrochloric acid.