Final answer:
Secretory proteins are examples of proteins that undergo cotranslational import as they are synthesized into the rough endoplasmic reticulum. Mitochondrial and nuclear proteins do not undergo cotranslational import, as their import into respective organelles happens post-translationally.
Step-by-step explanation:
Examples of proteins that undergo cotranslational import include secretory proteins. These proteins, as well as those destined for lysosomes, peroxisomes, or other microbodies, complete their elongation directly into the cisternae of the rough endoplasmic reticulum (RER). Cotranslational import refers to the process in which proteins are synthesized and simultaneously translocated into or across membranes.
Mitochondrial and nuclear proteins, however, do not undergo cotranslational import. Instead, mitochondrial proteins are typically post-translationally imported into the mitochondria after they have been fully synthesized and folded in the cytoplasm. Similarly, nuclear proteins are synthesized in the cytoplasm and then transported into the nucleus through nuclear pores.
Therefore, the correct answer to which proteins undergo cotranslational import is B) Secretory Proteins.