- In alginate impression materials, the gelation process is initiated by the interaction between alginate salts and calcium ions.
- Alginate is a naturally occurring polysaccharide derived predominantly from brown seaweed and exists in the form of sodium or potassium alginate in the impression material.

- The key component responsible for forming the gel is sodium alginate.
- When the powder containing sodium alginate is mixed with water, the sodium ions are replaced by calcium ions supplied by calcium sulfate (a separate component in the mix).
- This ion exchange causes the polymer chains of alginate to cross-link, resulting in the formation of a stable gel matrix.

To clarify the roles of the components:
- Sodium alginate: The principal reactive polymer that undergoes gelation upon exposure to calcium ions.
- Calcium sulfate: Serves as the source of calcium ions, but does not itself gel.
- Potassium alginate is less commonly used for impression materials.
- Zinc oxide does not participate in the alginate gelation process and is typically used in other dental materials.

Therefore, the correct answer is Sodium alginate, as it is the component that interacts directly with calcium ions to initiate gel formation.

Reference: "Craig's Restorative Dental Materials, 13th Edition, Chapter 9: Impression Materials, Page 250"