Exon is a part of gene that will become a part of final mature RNA produced by that gene after introns have been removed by RNA splicing.
Cistron is smallest unit of genetic material that must be intact to function as a transmitter of genetic information.
A  codon is a sequence of three DNA or RNA nucleotides that corresponds with a specific amino acid (or stop signal) during protein synsis.
Thus, correct answer is option (C).

Purines are a type of nitrogenous bases which are heterocyclic compounds. They are 9 membered and are double-ring structures. Nitrogen is present at positions-1, 3, 7 and 9 of ring. Adenine and guanine are only two examples of purines.