Answer: The correct answer for the fill in the blank is C. Pores.
There are specialized channels in the nuclear envelope( surrounding the nucleus of the cell). These channels are called as nuclear pore complexes or pores. They attach the inner nuclear membrane to the outer nuclear membrane and allow the passage of small molecules, which are polar in nature as well as larger macromolecules. Thus, through nuclear pore complex, transport of molecules take place between nucleus and cytoplasm.