becoz it does not follow the constraint or properties of multivalued dependency (x-->-->y)...
which is , if 2 tuples t1 and t2 exist in r relation such that t1(x)= t2(x) then 2 tuples t3 and t4 should also exist in r with
1) t3(x)=t4(x) =t2(x)=t1(x)
2)t3(y)= t1(y) and t4(y)=t2(y)
3) t3(z)=t2(z) and t4(z)=t1(z)
now in following question for x= d3 there is only 2 tuple say t1 and t2 , there should be t3 and t4 but not exist here thats why last tuples does not follow the properties .