A) forms the coding sequence for a polypeptide.
B) may carry part of a coding sequence for a polypeptide.
C) regulates how often the gene gets transcribed.
D) No function - it is untranslated.
Exons are any nucleotide sequences encoded by the gene that remains in the RNA after RNA splicing (removal of introns). RNA splicing is the process by which the non-coding regions, introns, are removed and the exons are covalently attached.
Hence, an exon in a gene is the set of sequences that forms the coding sequence for a polypeptide (protein) - letter A.
The image shows an example of a gene with its different regions. The introns are removed (spliced) and the exons are connected to be read for the polypeptide sequence.
Note that exons may refer to the DNA sequence within the gene or the corresponding sequence in the RNA.