The feature that can be used to store variables to control the style and behavior of a Lightning Web Component and ensure that the variables are testable in both Production and all Sandboxes is custom metadata. Custom metadata is data that describes other data and can be used to define the structure for application metadata. Custom metadata can be accessed in Apex, SOQL, validation rules, formulas, and flows. Custom metadata can also be deployed, packaged, and upgraded like other metadata. Custom setting is a type of custom object that can store data that applies to a specific organization, profile, or user. Custom setting data is cached and can be accessed in Apex, SOQL, validation rules, formulas, and flows. However, custom setting data is not included in change sets or packages and must be manually migrated to other environments. Custom object is a type of object that can store data that is specific to an organization or application. Custom object data can be accessed in Apex, SOQL, validation rules, formulas, and flows. Custom object data can also be deployed, packaged, and upgraded like other metadata. However, custom object data is not cached and may have performance implications when used to control the style and behavior of a Lightning Web Component. Custom variable is not a valid feature in Salesforce. Reference: [Custom Metadata Types], [Custom Settings], [Custom Objects]