I won't lie, going through the q packs and section bank... there are definitely a few really crappy questions. I remember this as one of them. I think this was a question where elimination of other answers was more useful.
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You can rule out C and D because genomic DNA won't indicate where splicing takes place.
B was tough. Working with mRNA is hard (cDNA is more stable). If you probe for exon 3, you'll get a result for the 16 residue (exons 1, 3, 4) but a blank result for the 17 residue (exons 1, 2, 4).
With option A you can directly compare two band sizes (in theory, I know 3bp would take a long time to resolve). The point is you design the experiment to get positive data when both isoforms are present - not just positive data when one isoform is present and negative data when the other one is present.
****ty question imo