Final answer:
Archaea and Bacteria are most similar in their ribosomal RNA sequence, despite differences in cell wall composition and plasma membrane lipids.
Step-by-step explanation:
Archaea and Bacteria are most similar in terms of their ribosomal RNA sequence. While these groups have fundamental differences, such as Bacteria having cell walls containing peptidoglycan and their plasma membrane lipids being composed of fatty acids, versus the lack of peptidoglycan in Archaea's cell walls and their plasma membrane lipids being formed from phytanyl groups, the ribosomal RNA (rRNA) is a point of similarity. Both Archaea and Bacteria have ribosomes for protein synthesis, and despite Archaea's ribosomal proteins being more similar to those in eukaryotic cells, the rRNA sequence is closely related between Archaea and Bacteria. Consequently, the correct answer to the question is ribosomal RNA sequence.