Yes, you can definitely use diagrams or illustrations to explain complex concepts in the answers for the Mechanical Engineering optional subject. Visual aids such as diagrams and illustrations can be extremely helpful in conveying complex information in a clear and concise manner. They can enhance the understanding of the reader and make the answer more visually appealing.

Using diagrams or illustrations in your answers can serve several purposes. They can help to:

1. Provide visual representation: Diagrams and illustrations can represent complex concepts in a simplified and visual manner. They can help the reader grasp the main ideas and visualize the relationships between various components or processes.

2. Clarify technical terms: Mechanical Engineering involves the use of technical terms that may be unfamiliar to some readers. By including diagrams or illustrations, you can visually explain these terms, making it easier for the reader to understand their meaning and significance.

3. Demonstrate processes or procedures: Mechanical Engineering often deals with various processes and procedures. By using diagrams or step-by-step illustrations, you can visually demonstrate these processes, making it easier for the reader to follow along and understand the sequence of events.

4. Highlight key points: Diagrams or illustrations can be used to highlight key points or important aspects of a concept. By visually emphasizing these points, you can draw the reader's attention to the most important information and help them remember the key takeaways.

To create visually appealing content, you can follow these guidelines:

- Use clear and neat diagrams or illustrations that are easy to understand.
- Label all the components or parts in the diagram for clarity.
- Use different colors or shading to differentiate different elements in the diagram.
- Use arrows or lines to indicate flow or movement.
- Keep the layout of the diagram organized and uncluttered.

Remember to keep the diagrams or illustrations relevant to the topic and use them to supplement your written explanation. They should enhance the understanding of the reader and provide additional insights into the complex concepts being discussed.