Among the three types of RNA—mRNA (messenger RNA), rRNA (ribosomal RNA), and tRNA (transfer RNA)—the largest is rRNA. This type of RNA plays a critical role in the formation of ribosomes, which are the cellular machinery responsible for protein synthesis.
To elaborate, rRNA comprises the structural and functional core of ribosomes. In eukaryotic cells, rRNA molecules can be quite large, with the 28S rRNA in the large ribosomal subunit being particularly substantial. Conversely, mRNA molecules can vary significantly in length, depending on the gene they encode, but they are generally shorter than rRNA. tRNA, on the other hand, is the smallest among the three, typically consisting of about 75-90 nucleotides.
In summary, when comparing these three types of RNA, rRNA is the largest due to its size and functional significance in ribosomal structure.