Jacob and Monod studied lactose metabolism in E. coli and proposed operon concept. Operon concept applicable for : -

An operon is a group of structural genes with a common promoter and operator that are transcribed together as one polycistronic mRNA, along with a regulator gene.
This type of gene regulation is characteristic of prokaryotes (e.g. lac, trp and ara operons of E. coli).
In eukaryotes, genes are generally transcribed separately (monocistronic) and regulated by more complex mechanisms; typical operons are not found (except in a few cases such as nematodes, which is not considered here).
So, as per the answer key, the operon concept is applicable to all prokaryotes.