"Researchers often want to isolate a certain type of RNA. For some RNA species, this can be accomplished via affinity chromatography, using beads coated with chains of poly-deoxythymidine (poly-dT). The desired RNA will stick to the beads while unwanted RNAs will flow through the column. The retained RNA can then be eluted."

The rest of the question from another source:

What RNA species can be purified using this method?

a. eukaryotic mRNA

b. eukaryotic rRNA

c. bacterial rRNA

d. bacterial mRNA

a. eukaryotic mRNA

Explanation:

Eukaryotic mRNA is modified through mRNA processing, by adding a poly-A tail to the newly formed mRNA. This involves the addition of a string of adenine nucleotides to the 3' end of the new mRNA, to protect it from cleavage in the cytoplasm. This does not happen in prokaryotes, or to rRNAs. Therefore, poly-dTs can be used to hybridise the polyA tail, and it can be used to specifically purify eukaryotic mRNAs.