Ans.
Gametes are defined as male or female germ cells, which fuse with gamete of opposite sex to form a zygote. Gametes are produced by the process of meiosis and thus, each gamete is haploid, having half the number of chromosomes as present in parent cell or in a normal cell.
When male and female gametes fuse together (sperm and egg, respectively), it leads to formation of zygote. As each gamete is haploid in nature, fusion of gametes results in formation of zygote having two sets of chromosomes (one from each gamete), hence, zygote is considered as diploid in nature.
Thus, the correct word to complete the sentence is 'diploid.'
