Intermediate goods are best described as:

Intermediate goods are essential in the production process as they are used to create final goods. These goods are not sold directly to consumers; rather, they are transformed during the manufacturing process. For example, steel is an intermediate good when used in the production of cars. This distinction is crucial for understanding how value is added in the supply chain from raw materials to finished products.

The other options do not accurately define intermediate goods. Finished goods are, by definition, ready for consumer use and do not require further production. Goods with a long lifespan may or may not be intermediates but are generally classified differently, such as durable goods. Lastly, goods that are not tradable have no relevance to the definition of intermediate goods, which can be produced and used in both domestic and international markets. Hence, identifying intermediate goods correctly is vital for comprehending production processes and economic analysis.