Final answer:
rRNA, or ribosomal RNA, is the nucleic acid that combines with proteins to form the subunits of ribosomes, essential for protein synthesis.
Step-by-step explanation:
The nucleic acid that combines with preexisting protein to form ribosomal subunits is rRNA, or ribosomal RNA. Ribosomal RNA is a major constituent of ribosomes, which are the cellular components responsible for protein synthesis. Ribosomal RNA is the most abundant type of RNA that combines with proteins and makes ribosomes. Ribosomes have two subunits: a large subunit and a small subunit. Ribosomes are the site for protein synthesis in cells. A ribosome has two parts: a large subunit and a small subunit, both of which include rRNA and proteins. The rRNA ensures the proper alignment of the mRNA (messenger RNA) and the ribosomes during protein synthesis. Additionally, rRNA has an enzymatic activity as it catalyzes the formation of peptide bonds between aligned amino acids during the creation of a protein.