Introduction:
In the field of materials science and engineering, composites are materials made of two or more constituent materials with significantly different physical or chemical properties. These constituent materials, called phases, are combined to create a material that possesses enhanced properties compared to its individual components. However, not all materials are composites. In this case, we need to determine which material among the given options is not a composite.

Analysis:
Let's analyze each option to determine if it is a composite material or not:

a) Wood:
Wood is an example of a natural composite material. It is composed of cellulose fibers embedded in a lignin matrix. The cellulose fibers provide strength and rigidity, while the lignin matrix acts as a binder. Therefore, wood is a composite material.

b) Concrete:
Concrete is a man-made composite material. It consists of cement as the binder matrix, aggregates such as sand and gravel for reinforcement, and water. The combination of these components creates a strong and durable material used in construction.

c) Plywood:
Plywood is a type of engineered wood product made from thin layers of wood veneer glued together with an adhesive. It is composed of multiple layers, or plies, with the grain direction of adjacent layers typically oriented perpendicular to each other. This cross-grain arrangement enhances the strength and stability of the material. Plywood is, therefore, a composite material.

d) Sialon:
Sialon (Silicon Aluminum Oxynitride) is a high-performance ceramic material. It is a solid solution of silicon nitride (Si3N4), aluminum nitride (AlN), and oxides of aluminum and silicon. Sialon does not consist of multiple phases or components, but rather it is a homogeneous material with a unique chemical composition. Therefore, it is not considered a composite material.

Conclusion:
Among the given options, the material that is not a composite is d) Sialon. Sialon is a homogeneous ceramic material, while the other options (a) Wood, b) Concrete, and c) Plywood) are all composite materials made of multiple phases or components.