Final answer:
Salesforce features may have metadata unique to an environment which can cause deployment errors, and Environment Variables are used to mitigate this problem.
Step-by-step explanation:
Sometimes, Salesforce features can contain metadata that are unique to a specific environment, such as the Id of a profile that is referenced in a validation rule or an integration endpoint. This can result in deployment errors as Salesforce is unable to locate the Ids in the destination environment. This is where Environment Variables come in handy. The correct answer is: a. Metadata, Environment, Ids, Environment Variables.