An example is next:
public Class Myclass (
public void Mymethod() { /* implementation */ }
}

B. Sharing rules will be inherited from the calling context.

If a class isn’t explicitly declared as either with sharing or without sharing, the current sharing rules remain in effect. Therefore, the class doesn’t enforce sharing rules except when it acquires sharing rules from another class. For example, if the class is called by another class that has sharing enforced, then sharing is enforced for the called class.


https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_keywords_sharing.htm

A is the answer