The correct option is C Cistron
A cistron is a gene. The term cistron is used to emphasize that genes exhibit a specific behaviour in a cis-trans test; distinct positions within a genome are cistronic. For example, an operon is a stretch of DNA that is transcribed to create a contiguous segment of RNA, but contains more than one cistron / gene. The operon is said to be polycistronic whereas ordinary genes are said to be monocistronic.