MicroRNAs (miRNAs) are endogenous small non-coding RNA molecules (~22 nt) ubiquitous in eukaryotic organisms that post-transcriptionally regulate mRNA expression by base-pairing with mRNA targets to trigger degradation or translational repression. All four statements are correct.

Evolutionary and fidelity rationale for RNA primers:

• Assertion (A) is true: De novo initiation of polynucleotide synthesis lacks proofreading and is inherently error-prone; initiating with a transient RNA primer ensures that low-fidelity initial segments do not become permanent features of the genome.
• Reason (R) is true: Because RNA contains ribose and uracil, it is recognized as temporary and excised by exonucleases, then replaced with proofread DNA to guarantee high-fidelity base pairing.
• Reason (R) provides the direct mechanism explaining why the use of an RNA primer prevents errors.
• Therefore, both (A) and (R) are true and (R) is the correct explanation of (A).

Function of RNA polymerase subunits:

• The $\beta'$ subunit contributes to the DNA template-binding cleft along with catalytic domains.
• Initiation is primarily coordinated by the sigma factor and core complex assembly.
• The core enzyme subunits $\beta$ and $\beta'$ form the catalytic center for ribonucleotide polymerization and processive phosphodiester bond synthesis; mutations in the $\beta'$ subunit impair RNA chain elongation, designated as the correct key answer by NTA.
• Promoter identification is mediated specifically by the sigma ($\sigma$) subunit.