Q.Higher organisms have resorted to sexual reproduction in spite of their complexity. Why?

Genetic variation among offspring is generated in sexual reproduction, through recombination, crossing-over and independent assortment during meiosis and gamete fusion.

In asexual reproduction (including its special case, parthenogenesis), offspring arise from a single parent by mitotic divisions, so they are essentially genetic clones of that parent -- the only source of difference would be rare, random mutations, not a designed mechanism for variation. Sexual reproduction, in contrast, involves meiosis (with crossing-over and independent assortment of chromosomes) in gamete formation, followed by fusion of gametes from two genetically different parents; this combination of processes resh …

Genetic variation among offspring is a hallmark of sexual reproduction, not asexual reproduction or parthenogenesis.

Asexual reproduction (option a) involves mitotic division of a single parent's cells, so the offspring are genetically identical clones -- no new variation is introduced. Parthenogenesis (option b) is development of an unfertilised egg into a new individual; since only one parent's genetic material is involved and no fertilisation occurs, offspring are also largely genetically identical to the parent, so option d ("both a and b") is also incorrect. Sexual reproduction, in contrast, involves meiosis (which introduces variation through crossing over and independent assortment of chromosomes) in both parents followed by random fertilisation (fusion of gam …