Catalyzes the formation of 4-diphosphocytidyl-2-C-methyl-D-erythritol from CTP and 2-C-methyl-D-erythritol 4-phosphate (MEP). 2-C-methyl-D-erythritol 4-phosphate + CTP + H(+) = 4-CDP-2-C-methyl-D-erythritol + diphosphate Isoprenoid biosynthesis; isopentenyl diphosphate biosynthesis via DXP pathway; isopentenyl diphosphate from 1-deoxy-D-xylulose 5-phosphate: step 2/6. Belongs to the IspD/TarI cytidylyltransferase family. IspD subfamily.