The "black box testing" methodology enforces what kind of restriction?

Knowing only the internal operation while being blind to external behavior does not describe any standard testing methodology and is the inverse of the correct answer.

Full knowledge of internal operation describes white box (or clear box) testing, not black box testing.

Partial access to internal operation describes gray box testing, which is a hybrid approach between black and white box testing.

In black box testing, the tester has no knowledge of or access to the internal implementation - only the external inputs and outputs are observable. This approach simulates real-world attack scenarios where an adversary has no insider knowledge of the system's architecture or source code. It tests the system purely from an external, functional perspective.