(c) : A nonsense mutation is the one which stops polypeptide synthesis due to the formation of termination or non­sense codon. In lac operon, sequence of structural genes is $Z, Y$, and $A$, which respectively code for $\beta$-­galactosidase, lactose permease and transacetylase. If the gene $Y$ has nonsense mutation, gene expression will stop at it, resulting in non­expression of both gene $Y$ and successive gene $A$. Thus, only $\beta$-­galactosidase enzyme will be produced.