Final answer:
The correct description of how a nucleus-bound protein enters the nucleus is through binding to a nuclear transport receptor, which then guides it through the nuclear pore in an energy-dependent process. Therefore, the correct option is D.
Step-by-step explanation:
The statement that best describes how a nucleus-bound protein gets into the nucleus is: It binds the nuclear transport receptor, which guides it through the nuclear pore. As the protein approaches a nuclear pore complex, it interacts with nuclear pore fibrils and the nuclear transport receptor. This facilitates the opening of the pore, and the nuclear transport receptor guides the protein complex across the double membrane of the nuclear envelope. The process of moving the protein into the nucleus against a concentration gradient is an active transport that relies on ATP hydrolysis as the protein enters the nucleus.