When a diploid female plant is crossed with a tetraploid male, the ploidy of endosperm cells in the resulting seed is :-

In angiosperms the endosperm is formed by triple fusion: one male gamete fuses with the two polar nuclei of the central cell.
Female plant is diploid (2n). Its megaspore and embryo sac are haploid, so each polar nucleus is n. Two polar nuclei together = n + n = 2n.
Male plant is tetraploid (4n). Its pollen and male gametes are formed after meiosis, so each male gamete is 2n.
Endosperm = polar nuclei + male gamete = 2n + 2n = 4n
So the endosperm cells are tetraploid (tetraploidy).
(The zygote would be n + 2n = 3n, i.e. triploid.)