The correct option is C cistron
Cistron- segment of DNA which determines the synthesis of the complete polypeptide. Thus, it is considered as equivalent to a structural gene. Eukaryotic structural gene is monocistronic whereas prokaryotic structural genes are polycistronic.
Muton- Smallest unit of DNA in which mutation occurs.
Operon- Functional unit of genomic DNA containing a cluster of genes under control of single promoter.
Recon- Smallest unit of DNA for recombination.