Final answer:
The correct answer is B. Proteins and large molecules, as they can cross the nuclear envelope through nuclear pores with the help of specialized transport proteins. Ions and small molecules can also pass through the pores, while lipids and carbohydrates as free entities typically do not.
Step-by-step explanation:
The structures and molecules that can cross the nuclear envelope are diverse and highly regulated. The nuclear envelope itself is a double-membrane structure, where each layer is a phospholipid bilayer. Embedded within this envelope are nuclear pores, which are large, rosette-shaped protein complexes that control the passage of materials.
Through these nuclear pores, a variety of substances can move between the nucleoplasm and the cytoplasm. Ions, small molecules, and RNA are able to pass relatively freely due to their size. Larger molecules, such as proteins and ribonucleoprotein particles (RNPs), require the assistance of special proteins that facilitate their movement. These transport proteins recognize specific signal sequences on the molecules that are destined for either import into or export out of the nucleus.
Lipids, being a part of the membrane structure, are integrated into the lipid bilayers and do not typically pass through pores. Furthermore, carbohydrates linked to proteins and lipids (glycoproteins and glycolipids) can be part of the membrane or elements within the nucleus or cytoplasm but do not directly pass through the nuclear pores as free molecules.