Final answer:
Horizontal gene transfer is a common phenomenon that can occur between unrelated organisms and is a significant source of genetic diversity. It can involve both prokaryotes and eukaryotes, affecting their evolution through natural selection.
Step-by-step explanation:
The correct answer to the question about horizontal gene transfer (HGT) is (d): it is common and may sometimes occur between unrelated organisms. Horizontal gene transfer can introduce genetic diversity within a single generation, as genetic material is transferred from one organism to another through various mechanisms such as transformation, transduction, and conjugation.
This transfer allows even distantly related species to share genes, influencing their phenotypes. HGT is more prevalent in prokaryotes and can occur between unrelated organisms. Although more prevalent among prokaryotes, HGT is not limited to them and can involve eukaryotes as well. It is an essential source of genetic variation which fuels the process of natural selection.