Ans. is 'd' i.e., All of the aboveThe fetus and the placenta contain paternally determined antigens which can lead to immunological rejection. The placenta has some role in preventing such a rejection:Placental hormones (EPF,PAPP-A,STERIODS & HCG) have got some immunosuppressive effect.There is production of blocking antibodies by mother in response to TLX (trophoblast lymphocyte cross reactive antigen) which protect the fetus from rejection.Absence of HLA molecules in villous trophoblast.