The correct option is D Number of the cells
The zygote or the early embryo upto 8 blastomeres (cells that are produced during cleavage of a zygote) is transferred into fallopian tube in zygote intra fallopian transfer (ZIFT) technique. If embryo contains more than 8 blastomeres and it is transferred into uterus than it is called intra uterine transfer (IUT). Hence, the major difference between ZIFT and IUT lies in number of cells.