Who explained the lac operon model for the first time?

The correct answer is Option (2) -Francois Jacob and   Jacque Monod.

In 1961, two French microbiologists,Francois Jacob and  Jacque Monod at the Pasteur Institute, Paris proposed a mechanism,called operon hypothesis, for the regulation of gene action as a result of their studies on the metabolism in E.coli. It got them Nobel prize in 1965. An operon acts as a gene-regulating unit.

In lac operon (here lac refers to lactose), a polycistronic structural gene is regulated by a common promoter and regulatory genes. Such arrangement is very common in bacteria and is referred to as operon.

The lac operon consists of one regulatory gene (the i gene – here the term i does not refer to inducer, rather it is derived from the word inhibitor) and three structural genes (z, y, and a).