Which of the following can pass through a nuclear pore?

The correct answer is both (a) and (c) (which corresponds to Ribosomal subunits and mRNA).

Step-by-Step Explanation:

1. What is a Nuclear Pore?
The nuclear envelope, which encloses the nucleus of a eukaryotic cell, is perforated by numerous protein complexes known as nuclear pore complexes (NPCs). These pores act as selective gateways that regulate the transport of macromolecules between the nucleus (where transcription and ribosome assembly begin) and the cytoplasm (where translation occurs).

2. Analyzing the transport of ribosomal subunits (Option a):
Ribosomal subunits are assembled inside the nucleolus (a region within the nucleus) from ribosomal RNA (rRNA) and ribosomal proteins. However, protein synthesis occurs in the cytoplasm. Therefore, these newly assembled, relatively large ribosomal subunits must be exported from the nucleus to the cytoplasm through the nuclear pores. Special transport receptors facilitate their passage.

3. Analyzing the transport of Mitochondria (Option b):
Mitochondria are double-membraned, independent organelles. They have a typical diameter ranging from 0.5 to 1.0 micrometer ($\mu \mathrm{m}$). On the other hand, the functional diameter of a nuclear pore is only about 10 to 40 nanometers ($\mathrm{n}\mathrm{m}$). Because mitochondria are hundreds of times larger than the nuclear pore opening, they cannot pass through nuclear pores.

4. Analyzing the transport of mRNA (Option c):
Messenger RNA (mRNA) is transcribed from DNA inside the nucleus. To be translated into proteins by ribosomes, the mRNA must be exported into the cytoplasm. Eukaryotic cells use specific export proteins (like exportins) to guide mature mRNA molecules safely through the nuclear pore complex into the cytoplasm.

Conclusion:
Since both ribosomal subunits (a) and mRNA (c) are actively transported through nuclear pores, while mitochondria (b) are far too large, the correct option is indeed both (a) and (c).