The equivalent of a structural gene is
(2) Cistron is the segment of DNA which determines the synthesis of complete polypeptide. Thus, it is considered as equiaveltn to a structural gene. Therefore, option (2) is correct and others are incorrect.
Concept Enhancer - Eukaryotic structural gene is monocistronic whereas prokaryotic structural gene is polycistronic.
Muton - Smallest unit of DNA in which mutation occurs.
Operon - Functional unit of genomic DNA containing a cluster of genes under condtrol of single promoter.
Recon - Smallest unit of DNA for recombination.