The correct answer is Sodium hypochlorite.

- Vital tooth bleaching involves the application of bleaching agents to teeth that are still alive (vital) in order to lighten their color.
- The most commonly used bleaching agents for vital tooth bleaching are carbamide peroxide, hydrogen peroxide, and sodium perborate due to their effective oxidizing properties and relative safety when used appropriately.

- Carbamide peroxide decomposes into hydrogen peroxide and urea, making it a widely used agent in both in-office and at-home bleaching systems.
- Hydrogen peroxide is a strong oxidizing agent that directly breaks down pigmented molecules within the tooth enamel.
- Sodium perborate is often combined with hydrogen peroxide in intracoronal bleaching, especially for non-vital teeth, but it can be cautiously used in certain vital bleaching protocols.

- In contrast, sodium hypochlorite is a strong chlorinating agent primarily used as an irrigation solution in endodontic therapy due to its ability to dissolve organic tissues and its potent antimicrobial properties. However, it is highly cytotoxic and irritating to soft tissues and can cause significant damage to the pulp and surrounding periodontal structures.

- Therefore, sodium hypochlorite is contraindicated for vital tooth bleaching because it can cause severe tissue irritation, damage the pulp vitality, and lead to adverse clinical outcomes.

In summary:
- Sodium hypochlorite should never be used for vital tooth bleaching due to its cytotoxicity and tissue-damaging effects.
- Carbamide peroxide, hydrogen peroxide, and sodium perborate are safe and effective bleaching agents for vital teeth when used properly.

Reference: Stuart C. White, Michael J. Pharoah, Oral Radiology: Principles and Interpretation, 7th Edition, Chapter 20 – Tooth Whitening, Page 530