Final answer:
The plantwide overhead rate is calculated using volume-related measures, making it suitable when overhead costs are correlated with these measures. It's most accurate when the relationship between overhead and volume is clear.
Step-by-step explanation:
The accuracy of allocating overhead costs using the plantwide overhead rate method is dependent on the firm's cost structure and activities. When opting for this method, answer choice a is correct: The rate is determined using volume-related measures. The rate is usually established based on a single measure like labor hours or machine hours, which are related to volume. Using the plantwide overhead rate is most appropriate when overhead costs have a clear correlation to these volume-related bases. In the case where overhead costs are not closely tied to such volume-related measures, using a plantwide rate might not be the most accurate approach and other methods, such as activity-based costing, could provide better cost allocation.