Question

If a male angiosperm plant is diploid and its female plant tetraploid, the ploidy level of endosperm will be

Answer 1

If the male plant is diploid, the microspore (pollen grain) produced by it will be haploid (as microspore is produced by meiosis from microspore mother cell). The pollen grain contains two sperm nuclei and each one is haploid.
When female plant is tetraploid, the embryo sac will be diploid (as embryo sac is produced by meiosis from megaspore mother cell). The embryo sac contains three antipodal cells, two synergids and two polar nuclei. Here each cell and nuclei are diploid.

Since the endosperm is produced by the fusion of one male sperm nucleus of pollen (here male sperm nucleus is haploid) and two polar nuclei of embryo sac (here each polar nuclei is diploid) . Thus, the fusion of haploid sperm nucleus with two diploid polar nuclei will form the pentaploid endosperm.