The correct option is B Splicing
The gene encoding for mRNA has two types of sequences called the introns and the exons. The introns are the intervening sequences and they do not code for a functional polypeptide.
The exons are the functional sequences. However, the exons are intervened by the introns. When such a gene is transcribed, the RNA so formed will have both these sequences.
Such an RNA is called the hnRNA (heterogeneous RNA) or the pre-mRNA. This pre-mRNA undergoes a post-transcriptional modification called splicing where these introns are removed from the pre-mRNA.
The RNA obtained after the process of splicing is called the mRNA and it has the sequences for only exons, that will be translated to form a functional polypeptide.
Thus, the correct answer is (A).