Which of the following is a restriction being enforced in "white box testing?"

This choice is nearly correct in direction but flawed - white box testing means the internal operation is fully known, not merely partly known, making 'only' an inaccurate qualifier.

In white box testing, the tester is given full visibility into the internal structure, source code, and design of the system. This allows the tester to craft test cases that exercise specific code paths, branches, and logic flows. The approach contrasts with black box testing, where no internal knowledge is provided.

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

Knowing only the external operation of a system - without any internal visibility - is the definition of black box testing, not white box testing.