The correct answer is option 'C', tergum.

The abdomen of a cockroach is composed of several segments, each covered by a dorsal plate of the exoskeleton. These dorsal plates are called terga.

Here is a detailed explanation of the answer:

1. Cockroach anatomy:
- Cockroaches have a three-part body structure consisting of the head, thorax, and abdomen.
- The abdomen is the posterior part of the body and is composed of multiple segments.

2. Exoskeleton:
- Cockroaches, like other insects, have an exoskeleton, which is a rigid outer covering that provides support and protection.
- The exoskeleton is made up of chitin, a tough and flexible material.

3. Dorsal plates:
- The dorsal plates are the upper plates of the exoskeleton that cover the dorsal (upper) side of the cockroach's body.
- In the abdomen, each segment is covered by a dorsal plate known as a tergum.

4. Tergum:
- The tergum is a specific term used to describe the dorsal plate of the exoskeleton found on the abdomen of a cockroach.
- It is also referred to as the tergite.
- The terga protect the internal organs and provide structural support to the abdomen.

5. Other options:
- Pleuron: The pleuron refers to the lateral (side) plates of the exoskeleton.
- Sternum: The sternum is the ventral (underside) plate of the exoskeleton.
- Vertex: The vertex is the uppermost part of an insect's head.

In conclusion, the dorsal plate of the skeleton found on the abdomen of a cockroach is called the tergum. It is one of the components of the exoskeleton that covers each segment of the abdomen, providing protection and support.