Final answer:
Archaea are more closely related to eukaryotes than to bacteria, with which they share a number of genetic and biochemical traits. They are unique single-celled organisms in their own domain, distinct from both bacteria and eukaryotes. Therefore, the correct option is A.
Step-by-step explanation:
The sentence that is true of archaea is A. They are more closely related to eukaryotes than to bacteria. Archaea are single-celled organisms that belong to their own unique domain of life. While they are prokaryotic like bacteria, meaning they lack a nucleus, archaea share more genetic and biochemical traits with eukaryotes. Archaea contain unique genes and proteins, as well as distinct metabolic pathways, which also support the idea of their close evolutionary relationship to eukaryotes rather than bacteria. This connection is further emphasized by the hypothesis that eukaryotes may have arisen from an ancient symbiosis between an archaeal and a bacterial cell.