Cytochrome P450 monooxygenase; part of the gene cluster that mediates the biosynthesis of the 6-methyl-2-pyrone derivative xylariolide D. XilC hydroxylates the 5-alkyl-6-methyl-2-pyrone backbone called prexylariolide D, produced by the highly reducing polyketide synthase xilA, on its side chain to form xylariolide D (Probable)