When a diploid female plant is crossed with a tetraploid male, what will be the ploidy of endosperm cells in resulting seed?

The correct option is A Tetraploidy
the diploid female produces embryo sac in which egg cell has n and polar nuclei have 2n (due to the presence of two nuclei) and tetraploid male (4n) produce pollen (2n). During the fertilization process, triple fusion i.e., the fusion of one male nucleus (2n) and two polar nuclei (2n) and produces endosperm with 2n + 2n = 4n. Thus, the correct answer is option A.