Which structure fuses form a diploid zygote during sexual reproduction

An egg cell's and a sperm cell's nucleus fuses with each other to form a diploid zygote

Structures are an egg and a sperm.

Explanation:

The sexually reproducing organisms have sex organs. The sex organs are of two types, male sex organs and female sex organ. The sex organs undergo meiosis to form male gamete by male sex organ and female gamete by female sex organs. The haploid male and female gamete fuse or fertilize to form diploid zygote. The zygote forms an embryo which develops into offspring.