Archaea are thought to have diverged from some ancient group of bacteria.

The correct answer would be gram-positive bacteria.

Explanation:

The gram-positive bacteria cell structure is most similar to the archaeal cell because they contain a thick exoskeleton of different chemical composition and both cells have a single lipid bilayer.

In phylogenetic trees, the archaeal homolog characters are more closely related to gram-positive bacteria. It has been suggested that the archaea diverged from gram-positive bacteria in counter-reaction to antibiotic selection pressure.

Thus, the correct answer would be gram-positive bacteria.