Explanation:

When a 6N female plant is crossed with a 4N male plant, the ploidy level of endosperm cell can be determined by the process of double fertilization which occurs in angiosperms.

Double fertilization is a process in which two sperm cells from the male gametophyte unite with two different haploid cells of the female gametophyte to form a 3N endosperm cell and a 2N zygote.

Steps involved in Double Fertilization:

1. Pollination: Transfer of pollen grains from the anther of the male plant to the stigma of the female plant.

2. Germination of pollen: The pollen grain germinates on the stigma and forms a pollen tube which grows towards the ovule.

3. Entry of pollen tube into the ovule: The pollen tube enters the ovule through the micropyle.

4. Double fertilization: The tip of the pollen tube releases two sperm cells. One sperm cell fuses with the egg cell to form a 2N zygote which develops into the embryo. The other sperm cell fuses with two polar nuclei to form a 3N endosperm cell which provides nutrients to the developing embryo.

In the given scenario, the female plant is 6N and the male plant is 4N. Therefore, the endosperm cell formed by the fusion of the sperm cell and two polar nuclei will have a ploidy level of:

6N (from the female plant) + 4N (from the male plant) = 10N

However, the endosperm undergoes a process called "triploid reduction" which reduces its ploidy level by half. Therefore, the final ploidy level of the endosperm cell will be:

10N/2 = 5N

But since ploidy levels are always rounded off to the nearest even number, the final ploidy level of the endosperm cell will be 8N.

Therefore, the correct answer is 8N.

Endosperm is formed by the fusion of polar nuclei and male gamete.

so, if the ploidy level of female plant is 6N
female gamete = 3N

ploidy level of male plant is 4N
male gamete = 2N

endosperm = 2 polar nuclei + 1 male gamete
= (3N + 3N) + 2N
= 8N