Question

signature sequences in 18s rrna molecules are ______.
common to all cellular organisms
found only in eukaryotes
found in eukaryotes and archaea
subject to horizontal gene transfer

Explanation:

18S rRNA is a component of the small ribosomal subunit specific to eukaryotic organisms. Signature sequences in this rRNA are unique to eukaryotes; prokaryotes (including archaea) have equivalent 16S rRNA instead, and these signature sequences are not subject to frequent horizontal gene transfer, nor are they shared across all cellular organisms.

Answer:

found only in eukaryotes