Final answer:
The statement is true; a plant asset's useful life represents the time it contributes productively to a company's operations, which is key for calculating depreciation.
Step-by-step explanation:
The statement that a plant asset's useful life is the length of time it is productively used in a company's operations is true. A plant asset typically refers to fixed assets like machinery, buildings, and equipment that are used in the production or selling of goods and services. The useful life of these assets is an estimation of the period over which the asset is expected to contribute to the company's operations, and this is often used for depreciation.