Final answer:
The addition of a poly-A tail to mRNA is a key post-transcriptional modification that increases mRNA stability and translational efficiency in eukaryotic cells.
Step-by-step explanation:
The type of eukaryotic post-transcriptional modification that enhances stability and translational efficiency of mRNA is the addition of the poly-A tail to the 3' end of pre-mRNA molecules. This modification not only protects the mRNA from degradation but also assists in the mRNA's export from the nucleus. In eukaryotic cells, the longevity of mRNA in the cytoplasm is crucial, as it dictates the time available for translation and determines the quantity of protein that will be synthesized. Other post-transcriptional mechanisms include RNA splicing, the attachment of a 5' cap, and regulation by microRNAs (miRNA).