Final answer:
CRISPR RNA (crRNA) is the small RNA type that is unique to prokaryotes.
Step-by-step explanation:
Out of the given options, the small RNA type that is unique to prokaryotes is the crRNA, which stands for CRISPR RNA. CRISPR RNA is a type of RNA that is involved in the immune system of prokaryotes, specifically in the defense against viral and plasmid DNA. It plays a crucial role in the CRISPR-Cas system, which is a prokaryotic adaptive immune system.