To transport a large protein (19 nm in diameter) into the nucleus, the structure that would be large enough to facilitate this process is the nuclear pore.
Nuclear pores are large protein complexes that span the nuclear envelope and allow the transport of molecules between the nucleus and the cytoplasm. They are capable of accommodating and facilitating the transport of large macromolecules, including proteins.
On the other hand, aquaporins are specialized channels for the transport of water and are not suitable for protein transport, and proton pumps are involved in the movement of protons across membranes, also not applicable for transporting proteins.
Therefore, the correct answer is nuclear pore.