Final answer:
The smallest type of RNA is microRNA (miRNA), which is smaller than mRNA, rRNA, and tRNA and functions in the regulation of gene expression.
Step-by-step explanation:
Among the major types of RNA, which include messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), and microRNA (miRNA), the smallest is microRNA (miRNA). mRNA is responsible for carrying the genetic code from DNA to the ribosome, where it is read in sets of three bases known as codons; each codon specifies a particular amino acid. Ribosomal RNA is a key component of ribosomes, ensuring the proper alignment of mRNA and playing a role in peptide bond formation during protein synthesis. Transfer RNA is relatively small, usually consisting of 70-90 nucleotides, and it carries the correct amino acid to the site of protein synthesis. However, microRNAs, which function in regulating gene expression by interfering with the expression of certain mRNA messages, are considered the smallest RNA molecules.