Male gametes in diploid maize plants have 10 chromosomes in their nucleus. What would be the number of chromosomes in its female gamete, zygote and the cells of the seedling respectively?

The correct option is B 10, 20, 20
Chromosome number in male gamete = 10.

Therefore the number of chromosome in female gamete will also be 10.
Since the plant is diploid, zygote will be 2n (n = number of chromosomes in the haploid set) as it is formed from the fusion of two haploid gametes.
2n = 2x10 = 20
Seedlings are the product of division and differentiation of zygote so it will also be diploid which, here is 20.