Which one of the following is a triploid cell ?

In flowering plants the endosperm forms after double fertilization.

One sperm nucleus fuses with the egg cell to produce the diploid zygote (2n). The second sperm nucleus fuses with the central cell, which already contains two haploid polar nuclei (each n).

Thus the primary endosperm cell results from the combination:

n (sperm) + n (polar nucleus 1) + n (polar nucleus 2) = 3n.

Because it contains three sets of chromosomes, the primary endosperm cell is triploid.

For comparison:

- Central cell before fertilization is diploid (2n) because it holds two haploid polar nuclei.

- Zygote is diploid (2n) from egg (n) + sperm (n).

- Synergid cells are haploid (n) and do not participate in fertilization.

Therefore, among the given options, the only triploid cell is the Primary endosperm cell.