There is no natural death in single celled organisms like Amoeba and bacteria because

Answer: (c) parental body is distributed among the offspring

When a single-celled organism such as Amoeba or a bacterium divides by fission, the whole parent cell becomes the two (or more) daughter cells. The parent does not leave behind a dead body; it simply continues to live as its offspring. That is why there is no natural death in such organisms.

Binary fission (b) is only the method of division; the real reason is that the parent body itself is shared among the offspring. Being microscopic (d) or asexual (a) does not explain it.