Catalyzes the CTP-dependent phosphorylation of riboflavin (vitamin B2) to form flavin mononucleotide (FMN). CTP + riboflavin = CDP + FMN + H(+) Binds 1 Mg(2+) ion per subunit. Cofactor biosynthesis; FMN biosynthesis; FMN from riboflavin (CTP route): step 1/1. Belongs to the archaeal riboflavin kinase family.