The correct option is C Antibiotic resistance gene
A gene carrying antibitoic resistance like ampicillin or kanamycin are inserted into the plasmid vector and the organisms are grown in media containing antibitoic so that in theory, only those with the gene for antibiotic resistance will survive and hence can be selected.
Replicon consists of origin of replication site (ori site) and other controlling genes that helps in the replication of the plasmid. For a plasmid to be viable vector, it needs to be self replicating. The replicon in the plasmid is responsible its replication. The gene of interest has the gene to be inserted into the vector and plays no role in selection.