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

The correct option is B

tetraploidy

The correct option is B.

Explanation of the correct option:

1. When the male plant is tetraploid then the gamete generally formed after meiotic division is diploid.
2. The female plant = diploid therefore the gamete formed after meiotic division will be haploid.
3. The endosperm cell is usually formed by the fusion of two polar nuclei and one male gamete (double fertilization).
4. The 2 polar nuclei i.e. $\left(n+n\right)=2n$ whereas one male gamete $=2n$, which fuses to form the endosperm cell $=4n$ or tetraploid.

Explanation of the incorrect option:

Option A: Pentaploidy is having five times the haploid number of chromosomes.

Option C: Triploidy is having three times the haploid number of chromosomes.

Option D: Diploidy is having two times the haploid number of chromosomes.

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