Final answer:
The critical length of the polyA tail is reached when about 200 adenine residues are added, making option B) 200-300 nucleotides the correct answer.
Step-by-step explanation:
The critical length of the polyA tail in eukaryotic mRNAs is achieved when the enzyme poly-A polymerase adds a string of approximately 200 adenine residues to the 3' end of the pre-mRNA. Therefore, the correct answer to the question is B) 200-300 nucleotides. This modification plays a crucial role in protecting the pre-mRNA from degradation, signaling the need for the transcript to be exported to the cytoplasm, and regulating the half-life of mRNAs in the cytoplasm.