Final answer:
Regulatory small RNAs, such as microRNAs, are typically 21-24 nucleotides in length, and their primary function involves regulating gene expression by binding and degrading target mRNA.
Step-by-step explanation:
Regulatory sRNAs, including microRNAs (miRNAs), are typically 21-24 nucleotides long. These short RNA molecules are processed in the nucleus from longer pre-miRNAs and then chopped into mature miRNAs by a protein called Dicer. Once formed, miRNAs bind to mRNAs with the help of a ribonucleoprotein complex known as the RNA-induced silencing complex (RISC), leading to the degradation of the target mRNA. miRNAs play a crucial role in the regulation of gene expression by controlling the stability of RNA molecules. The correct answer to the student's question is (a) 20-30 nucleotides long.