Major RNAs are not a category of RNAs. Long non-coding RNAs are non-protein coding transcripts typically longer than 200 base pairs (bp) and play a role in regulating gene expression and epigenetic regulation. MicroRNAs are small RNAs (~20 bp) and play a role in RNA silencing and post-transcriptional regulation of gene expression. Short interfering RNAs are double-stranded (20-25 bp) and play a role in post-transcriptional gene silencing. Piwi-interacting RNAs are small non-coding RNAs that interact with piwi proteins in epigenetic and post-transcriptional silencing of genetic elements such as retroposons. 

Only 2-5% of the total RNA content in a cell is mRNA. Approximately 10% is transfer RNA (tRNA), and approximately 85% is ribosomal RNA (rRNA).

The poly-A tail adds to the stability of the mRNA transcript. The tail becomes shorter and shorter over time due to exonucleases, eventually signaling an enzyme to break down the mRNA to stop further translation. The 5' methylated guanosine cap facilitates binding of ribosomes, and specific splicing sequences signal removal of introns, not the poly-A tail. The tail cannot signal the end of transcription by the RNA polymerase because the poly-A tail is a post-transcriptional change, meaning it is not present during transcription. Lastly, the poly-A tail lies at the end of the 3' UTR (untranslated region), and is not included in the protein product of the mRNA.

Introns are regions included in genes that are not actually part of the final protein generated. Scientists first observed that some areas of genes are removed before mRNA translation by visualizing that not all of a gene hybridizes with its cognate mRNA, and hence there are pieces that are spliced out and not used. Note that splicing of introns, like all other post-transational modifications, only occurs in eukaryotes. The function of intron regions is thought to be mostly regulatory.

The correct answer is repressors bind splicing silencers and increase nearby splice junction activity. Activators bind splicing enhancers and decrease likelihood of proximal sites as splice junction. These enhancer sites can be in the intron or exon of the nascent RNA molecule and are most commonly bound by serine and arginine (SR) proteins. Moreover, the relative abundance of these bound RNA regulatory elements in proximity to a splice junction confers differential splicing activity. 

The correct answer is all of the other answers. Exon skipping is the most common mode in mammals and occurs when an exon is spliced out of the primary transcript. Mutually exclusive exon splicing occurs when one of two exons is retained, but not both. Alternative donor site occurs when an alternative 5' splice junction is used which will change the 3' end of the upstream exon. Alternative acceptor site occurs when there is an alternative 3' split junction and the 5' end of the downstream exon is changed.