Global Access Modifier:
When building managed packages, methods and classes that need to be accessible outside the package namespace must be declaredglobal.
Publicrestricts access to within the namespace.
Why Not Other Options?
B:Publiclimits access to within the namespace.
CandD: Mixingpublicandglobalincorrectly doesn't meet the requirement for external accessibility.
References:Managed Packages and Global Access:https://developer.salesforce.com/docs/atlas.en-us.apexcode.
meta/apexcode/apex_classes_access_modifiers.htm