The primary endosperm nucleus is

The correct option is triploid.

To understand why the primary endosperm nucleus (PEN) is triploid, let us look at the process of double fertilization in flowering plants (angiosperms) step-by-step:
1. Double Fertilization: Flowering plants undergo a unique process called double fertilization in which two male gametes (sperm cells) enter the embryo sac.
2. First Fertilization (Syngamy): One haploid ($n$) male gamete fuses with the haploid ($n$) egg cell to form a diploid ($2n$) zygote, which later develops into the embryo.
3. Second Fertilization (Triple Fusion): The second haploid ($n$) male gamete fuses with the diploid polar nuclei. Specifically, it fuses with the central cell containing two haploid polar nuclei ($n+n=2n$).
4. Formation of the PEN: The fusion of these three haploid nuclei ($n+n+n$) is called triple fusion. The resulting nucleus is the primary endosperm nucleus (PEN). Since it is formed by the fusion of three haploid sets of chromosomes, its ploidy level is triploid ($3n$).
5. Function: The triploid primary endosperm nucleus subsequently undergoes multiple mitotic divisions to develop into the endosperm, a nutrient-rich tissue that provides nourishment to the developing embryo.