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

Diploid female plant forms haploid polar nuclei which fuse and form diploid secondary nucleus. The tetraploid male plant can form diploid male gametes. When diploid secondary nucleus fuses with diploid male gamete, there is formation of tetraploid primary endosperm nucleus which forms tetraploid endosperm.