Final answer:
Option A is correct option. The three post-transcriptional modifications of mRNA in eukaryotes occur in the nucleus, which includes 5' capping, 3' polyadenylation, and splicing. These prepare mRNA for translation in the cytoplasm.
Step-by-step explanation:
The three post-transcriptional modifications often seen in the maturation of mRNA in eukaryotes are 5' capping, 3' polyadenylation, and splicing. These modifications occur within the nucleus of the cell. After these modifications, the now mature mRNA is transported from the nucleus to the cytoplasm where it can be translated into a protein by the ribosomes. The correct answer to which cellular organelle these modifications occur in is A) Nucleus.
Other aspects of RNA and protein synthesis include the synthesis of rRNA in the nucleolus, the assembly of proteins on ribosomes in the cytoplasm or on the rough endoplasmic reticulum, and subsequent modifications and sorting in the Golgi apparatus before being distributed via vesicles.