What is the correct sequence of structural genes in lac operon ?

lac Z , lac Y, lac A .

The correct answer is Option (3)- lac Z , lac Y, lac A.

In lac operon (here lac refers to lactose), a polycistronic structural gene is regulated by a common promoter and regulatory genes. 

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 in the sequence of z, y, and a.The i gene codes for the repressor of the lac operon.

The z gene codes for beta-galactosidase ( β-gal), which is primarily responsible for the hydrolysis of the disaccharide, lactose into its monomeric units, galactose and glucose. The y gene codes for permease, which increases permeability of the cell to β-galactosides. The a gene encodes a transacetylase. Hence, all the three gene products in lac operon are required for metabolism of lactose.