Which of the following is mandatory to create a plant in SAP Materials Management?

To create a plant in SAP Materials Management, the language key is a mandatory field. The language key defines the language in which the plant's data will be displayed and accessed within the SAP system. This element is crucial as it ensures that the communication and information regarding the plant are consistent and understandable for the users according to their language preferences.

While other factors like division, local currency, and tax code are important for various functions and reporting within the SAP environment, they are not strictly essential for the initial creation of a plant. The division is typically tied to organizational structure, the local currency relates to financial transactions, and the tax code is relevant for compliance and tax management, but they are not prerequisites for establishing a plant in the system. Hence, the requirement of a language key signifies its critical role in the identification and functionality of the plant within SAP's framework.