Durable goods and industrial items are products that are designed to last for a long time and are usually used in the production of other goods or in industrial processes. These items are typically purchased by businesses rather than individual consumers. When it comes to the market for durable goods and industrial items, they primarily exist in the national market, which is option C in this case.

Here's an explanation of why the correct answer is option C:

1. Definition of durable goods and industrial items:
- Durable goods: These are products that have a long lifespan and are not consumed immediately after purchase. Examples include machinery, vehicles, furniture, and appliances.
- Industrial items: These are goods that are used in the production or manufacturing process. Examples include raw materials, components, and equipment.

2. Local market:
- The local market refers to a specific geographical area, such as a city or town.
- Durable goods and industrial items may be available in the local market to a certain extent, but the range of options and availability may be limited.
- Local businesses may source some of their durable goods and industrial items from nearby suppliers, but they may also need to look beyond the local market for a wider selection.

3. Regional market:
- The regional market refers to a larger geographical area, such as a state or province.
- While the regional market may offer more options and suppliers compared to the local market, it may still not be sufficient for businesses' needs when it comes to durable goods and industrial items.
- Businesses operating in the region may need to source these items from suppliers in other regions to meet their requirements.

4. National market:
- The national market encompasses the entire country or nation.
- This is where a wide range of suppliers, manufacturers, and distributors exist for durable goods and industrial items.
- Businesses can access a larger pool of options, compare prices, and choose the most suitable suppliers for their needs.
- The national market provides economies of scale, allowing businesses to benefit from lower costs and greater efficiency.

In conclusion, durable goods and industrial items primarily exist in the national market. While they may be available to some extent in the local and regional markets, the national market offers a wider range of options and suppliers for businesses to choose from.