The Cast Iron which possesses all the carbon in the combined form as cementite is known as

The correct option is White Cast Iron.

To understand why this is the correct answer, let's break down the classification of cast iron based on the form in which carbon exists within its microstructure:

1. White Cast Iron:
In white cast iron, all the carbon is present in the combined form as iron carbide (also known as cementite, ${\mathrm{Fe}}_{3}C$). This is typically achieved through rapid cooling during solidification, which prevents the carbon from graphitizing. Because cementite is extremely hard and brittle, white cast iron has a high hardness, excellent wear resistance, and shows a crystalline, "white" fractured surface when broken.

2. Grey Cast Iron:
In grey cast iron, most of the carbon is present in the free form as graphite flakes. When fractured, it displays a greyish appearance due to the exposed graphite. This type is characterized by good machinability and excellent dampening capacity.

3. Spheroidal Cast Iron (Ductile Iron):
In spheroidal or ductile cast iron, carbon is present in the form of spherical graphite nodules (spheroids). This shape is induced by adding small amounts of magnesium or cerium, which prevents the formation of sharp flakes, thereby improving ductility and toughness.

4. Malleable Cast Iron:
Malleable cast iron is produced by heat-treating white cast iron. The cementite is decomposed into irregular, nodular clusters of graphite (temper carbon), combining the casting benefits of cast iron with improved ductility.

Therefore, the cast iron that possesses all its carbon in the combined form as cementite is uniquely identified as White Cast Iron.