The correct option is B exons
Transcription is the process of synthesis of RNA from DNA.
In eukaryotes, the primary product of transcription is heterogeneous nuclear RNA (hnRNA) and it contains both exons and introns. Exons and introns are absent in prokaryotes.
Exons are the coding sequences that code for amino acids. Introns are the non-coding sequences that do not code for amino acids.
RNA splicing involves removal of the introns and simultaneous joining of the exons in a definite order. This leads to the formation of the mature mRNA.