**Core Concept**
Calcium sulfate, a compound commonly found in dental materials, undergoes different transformations when exposed to heat and pressure, resulting in various polymorphs or crystalline forms. The question pertains to the specific process of calcining gypsum under steam pressure or dehydrating it in the presence of sodium succinate.

**Why the Correct Answer is Right**
Alpha-hemihydrate is a specific form of calcium sulfate that is obtained through the calcination of gypsum under steam pressure at 120-130°C. This process involves the controlled dehydration of calcium sulfate dihydrate (gypsum) to produce a more stable, crystalline form of calcium sulfate. The presence of sodium succinate acts as a seeding agent to facilitate the formation of alpha-hemihydrate crystals. The resulting product is characterized by its unique crystal structure and is used in various dental applications, including the production of dental stones and models.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because it is the correct answer, but we will discuss the other options to clarify their inaccuracy.
**Option B:** Beta-hemihydrate is another form of calcium sulfate that is obtained through the calcination of gypsum at higher temperatures (around 150-160°C). It has a different crystal structure compared to alpha-hemihydrate.
**Option C:** Calcium sulfate dihydrate, also known as gypsum, is the starting material for the production of alpha-hemihydrate, but it is not the product itself.
**Option D:** Orthorhombic anhydrate is another polymorph of calcium sulfate, but it is not related to the process described in the question.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the specific form of calcium sulfate used in dental applications can significantly affect the properties of the final product, including its density, strength, and durability. Understanding the different forms of calcium sulfate and their production processes is crucial for dental professionals to select the most suitable materials for their specific needs.

**✓ Correct Answer: A. Alpha - hemihydrates**