the right answer is ribosomal RNA.
William E. Balch and Al in the publication in 1977 about the phylogenetic tree based on the ribosomal RNA gene sequences of the organisms studied, a tree in which prokaryotes were divided into two distinct domains, that of bacteria and that of archaea.
And we know that eucaryotes and procaryotes have different ribosomal RNA, for example, the ribosomes of the procaryotes are 70s (Svedberg), and the ribosomes of the eucaryotes are 80s.