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

Lets break this question a bit.

We know that during gamete formation half of the chromosomes are transmitted to the next generation.

So in case of the tetraploid male plant (4n), the gamete will have half the number of chromosomes i.e. 2n

Similarly in case of the hexaploid female plant (6n), the gamete will have have the number of chromosomes again i.e. 3n

Therefore upon fusion of gametes containing 2n and 3n chromosomes, the offspring gets a ploidy level of 2n+3n=5n.