To make Apex code, such as a class or method, accessible to another namespace (e.g., from a managed package to a subscriber's org), it must be declared with the global access modifier. The public access modifier only allows access within the same namespace. For the calculateBodyFat() method to be accessible outside the package, both the containing Bodyfat class and the method itself must be declared as global.

B. Declare the class and method using the public access modifier: The public modifier restricts visibility to within the package's namespace, making it inaccessible to the consumer's code.

C. Declare the class as public and use the global access modifier on the method: This is an invalid declaration. A method with global visibility must be contained within a global class.

D. Declare the class as global and use the public access modifier on the method: While the class would be visible, the public method would remain inaccessible outside the package's namespace.

1. Apex Developer Guide, "Access Modifiers". The documentation states, "The global access modifier declares that this class is known by all Apex code everywhere... If you declare a method or variable as global, you must also declare the class that contains it as global."

2. ISVforce Guide, "Apex Code in Packages". This guide details how access modifiers work in the context of managed packages, specifying that global is required for exposing an API.