$12.$ The position effect variegation $($PEV$)$ phenotype can be used to identify new genes that regulate heterochromatin formation. For instance, strains of Drosophila melanogaster with the White variegation phenotype have been subjected to mutagenesis to screen for dominant mutations $($in other genes$)$ that either enhance or suppress PEV, meaning the mutations result in either lower or higher red pigment production, respectively. Which of the following mutations is expected to be an enhancer of variegation? a$.$ A gain$-$of$-$function mutation in a gene encoding a histone methyl transferase that trimethylates lysine $9$ on histone H$3,$ resulting in a hyperactive form of the enzyme b$.$ A gain$-$of$-$function mutation in a gene encoding a histone acetyl transferase that normally acetylates lysine $9$ on histone H$3,$ resulting in higher expression of the protein c$.$ A mutation that results in the loss of function of the fly$$s HP$1($heterochromatin protein $1)$ gene d$.$ A loss$-$of$-$function mutation in a gene encoding a histone deacetylase that deacetylates lysine $9$ on histone H$3$