Answer
In unicellular organisms generally, asexual mode of reproduction is used, these are fission, budding, spore formation, etc. Where the offspring produced are identical to the parent.
In multicellular organisms, the sexual mode of reproduction is used. In this method, the male and female reproductive units are developed in separate organs respectively.
These units fuse together to form zygote which later develops into an offspring. The offspring has characters of both the parents and has more ability to cope the harsh environment as compared to its parents