The correct statements that compare a eukaryotic cell and a virus from the provided options are:
a A eukaryotic cell responds to changes in the environment, whereas a virus cannot adapt.
b A eukaryotic cell can reproduce to make more cells, whereas a virus cannot reproduce.
d A eukaryotic cell maintains homeostasis, whereas a virus does not possess the required organelles.
However, option c is incorrect because viruses do contain genetic material (DNA or RNA).
Thus, based on the comparisons offered, the strongest responses are a and b. However, b is particularly clear and fully captures the notable difference between the two. Therefore, b is the most appropriate answer.