Industrial applications of hydrochloric acid (HCl): What is the principal combined industrial use of HCl among the following options?

Introduction / Context:
Hydrochloric acid is a major inorganic commodity chemical. Recognizing its principal uses helps in plant design, storage logistics, and safety management.

Given Data / Assumptions:

• Common HCl uses include steel pickling, oil well acidizing, pH control, and inorganic salt production.
• The question lists one correct combined application widely practiced at scale.

Concept / Approach:
HCl dissolves scale and oxides from steel surfaces prior to cold rolling or coating (pickling). In petroleum well acidizing, aqueous HCl dissolves carbonate scales and enlarges flow channels to restore permeability. These are hallmark industrial uses of HCl.

Step-by-Step Solution:
Identify mainstream uses → steel pickling and oil well acidizing.Match with option → (a) explicitly lists both.

Verification / Alternative check:
Industry summaries list steel pickling as one of the largest consumers of HCl; oilfield services regularly use inhibited HCl for acid stimulation.

Why Other Options Are Wrong:

• (b) HCl is not a primary raw material for cationic detergents; organics and quaternization chemistries are used.
• (c) Spent fuel treatment involves nitric acid in PUREX; HCl is not central.
• (e) Oxidative bleaching relies on chlorine dioxide, peroxide, ozone—HCl is not the bleaching agent.

Common Pitfalls:
Assuming all acids are interchangeable across heavy industry; specific chemistries dictate acid choice.

Final Answer:
Acidizing of petroleum wells and pickling (descaling) of steel sheets