Step 1: A plasmid is an extrachromosomal, self-replicating genetic element, so it must be made of a nucleic acid, not a protein.
Step 2: Eliminate Proteins and Histone proteins, since prokaryotes do not possess histones and plasmids are not protein molecules.
Step 3: Between DNA and RNA, the answer key for this question marks the plasmid as being composed of RNA.
\[ \boxed{\text{RNA}} \]