**Core Concept**
Calcium alginate impression material is a type of hydrocolloid used in dentistry for making precise dental impressions. The addition of calcium salts to the alginate mixture during mixing is a crucial step that affects the material's properties and setting reaction.

**Why the Correct Answer is Right**
When calcium is added to the alginate mixture, it triggers a chemical reaction that leads to the gelation of the alginate. This reaction involves the ionization of calcium ions (Ca2+) and the subsequent interaction with the alginic acid molecules in the alginate. The calcium ions bind to the alginic acid, forming a cross-linked network of molecules that gives the impression material its gel-like consistency. This cross-linking process is essential for the material's setting and stability.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is typically incorrect, as the addition of calcium does not affect the alginate's sol-gel transition temperature or the initial setting time.
* **Option B:** This option is incorrect because the primary function of calcium is to initiate gelation, not to increase the material's fluidity or reduce its viscosity.
* **Option C:** This option is incorrect because calcium does not have a direct effect on the alginate's elasticity or flexibility.

**Clinical Pearl / High-Yield Fact**
When working with calcium alginate impression material, it's essential to mix the calcium powder with the alginate powder in a specific ratio to achieve the desired gelation time and material properties.

**Correct Answer:** D.