Question

which unit to form a diploid zygote and afterwards it develops into human embryo

Answer 1

Male gamete and female gamete unite to form a diploid zygote and afterwards it develops into human embryo. sperm and ovum fuse together to form a zygote and then developed into embryo andfoetus

Answer 2

Male gamete called sperm, and female gamete called ovum units forms  a diploid zygote and afterwards it develops into human embryo.

Explanation:

As soon as the mating process is completed, the male gamete which is haploid in condition is fused with female gamete which is also haploid in condition.

This leads to fertilization where both the gametes unites with one another and leads to formation of zygote which is diploid in condition and after certain stage of development it gets developed into embryo.