I do not understand how to solve this question. I know that RNAse will cut smaller pieces of RNA.
The answer given is A
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The question is a bit vague in some important parts, so I'll have to make a few assumptions about what the authors likely meant.
RNAses are enzymes that degrade RNA. There are a few different ones that lead to different kinds of degradation. The type that you would use in an experiment like this is an RNAse that completely degrades RNA. The purpose of this is to be able to distinguish RNA from DNA. So whatever is left after the RNAse treatment is DNA, and the bands that vanished are RNA.
If you isolate nucleic acids from liver, the major DNA part would be genomic DNA. The major RNA components would be mRNA and rRNA. Genomic DNA is huge compared to mRNA and rRNA. In gel electrophoresis of nucleic acids, the bands that move further are smaller nucleic acids, and the bands that move less far are larger nucleic acids. So the band on the left in your example is likely genomic DNA, and the band to the right is mRNA or rRNA. After RNAse digestion only the left band should remain, which is answer A.