Q.Sequences of which of the following is used to know the phylogeny

Step 1. Why a phylogenetic marker needs to be slowly-evolving. To compare how closely related two organisms are, biologists need a molecule that every organism has, that does the same essential job in every organism, and that changes only slowly over evolutionary time - so that the number of differences between two organisms' copies of it roughly tracks how long ago they last shared a common ancestor.

Step 2. Why mRNA fails this test. Messenger RNA (option a) is a poor choice: it is transcribed fresh from whichever gene is being expressed at a given moment, so its sequence and even its presence varies constantly with the cell's activity, gene by gene - it is not a single, stable, universally comparable molecule.

Step 3. Why tRNA is a weaker choice than rRNA. Transfer RNA (option c) is small, essential and reasonably conserved, but a cell carries many different tRNA molecules (one family per amino acid/codon), which makes a single, clean, genome-wide comparison harder than using one universal, large, information-rich molecule.

Step 4. Why HnRNA fails this test. Heterogeneous nuclear RNA (option d) is simply the unprocessed precursor transcript found only in the eukaryotic nucleus before splicing into mature mRNA - it is not even present in prokaryotes, so it cannot serve as a universal comparison tool across all of life.

Step 5. Why rRNA is the standard choice. Ribosomal RNA (option b), by contrast, is present in every living cell (since every cell needs ribosomes to make protein), performs the same essential structural and catalytic role everywhere, and evolves extremely slowly because almost any change to it is likely to disrupt protein synthesis and so gets selected against. This combination - universal, essential, slowly changing - is exactly what let Carl Woese use rRNA sequence comparisons (16S/18S rRNA) to build the modern three-domain view of life (Bacteria, Archaea, Eukarya), which is the same reasoning that makes rRNA the sequence of choice for phylogenetic study generally.

Test each RNA type against the requirements of a good phylogenetic marker - universally present, essential, and slowly evolving.

• Assuming mRNA carries the 'genetic' information most directly and is therefore the natural choice, without considering how much its sequence and abundance vary gene-by-gene and moment-by-moment.
• Forgetting that HnRNA is an exclusively eukaryotic, nuclear precursor and so cannot function as a universal cross-kingdom comparison tool.