When a diploid female plant is crossed with a tetraploid male, then find the ploidy of endosperm cells in the resulting seeds. Explain your answer.

When we cross a diploid female plant (2n) with a tetraploid male (4n), the endosperm formed will be tetraploid. This can be explained as follows:
Parents: 2n (Diploid female) ×​ 4n (Tetraploid male)

Gametes: n (Female gamete) × 2n (Male gamete)

Zygote: 3n

Endosperm is formed by the process of triple fusion, wherein one male gamete fuses with the two polar nuclei. In this case, the two polar nuclei are haploid but the male gamete is diploid (2n). Hence, the endosperm would be tetraploid (n + n + 2n = 4n).