I can only use the process of elimination, but the only one that seems to make sense is option B, as often the cells in the walls of the xylem die and are lignified, so I'd hazard a guess at option B.
C and D are not true; most eukaryotes and bacteria are unicellular therefore programmed cell death means death of the organism.
I can't see any logic behind why it would be A, as it's the expression of genes which control the differentiation of cells.
