Question

which is most critical event in sexual reproduction

Answer 1

fertilisation of gametes is the most critical event in sexual reproduction

Answer 2

The most important event in sexual reproduction is the fusion of the male gamete(sperms) and the female gamete(ova) in order to produce a single-celled zygote. 
This is indeed a critical event because the fusion and fertilization of gametes is a matter of chance and can't be pre-determined by anyone. out of all the sperms, only a single healthy sperm fertilises the egg. However, it may also be possible that the sperm fails to fertilse the egg.

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