All of the following are part of an operon except

Except for enhancer, all the given components are parts of an operon. Enhancer sequences are present in eukaryotes. When bound by specific proteins or transcription factors, these sequences enhance the transcription of an associated gene.
On the other hand, an operon is a regulatory unit of DNA containing a cluster of genes in prokaryotes.
The promoter of the operon is the site where RNA polymerase binds. The operator acts as an on-off switch to control transcription. The structural genes code for enzymes involved in metabolic pathway.