Primary Endosperm Nucleus is the product of :

Triple fusion

The correct answer is Option (2) – Triple fusion

After the pollen tube enters one of the synergids in the embryo sac, it releases two male gametes into the cytoplasm of the synergid. One of the male gametes moves towards the egg cell and fuses with its nucleus, resulting in syngamy or the fusion of two haploid nuclei. This fusion gives rise to a diploid cell called the zygote,  which will develop into an embryo.

The other male gamete moves towards the central cell, which contains two polar nuclei. The male gamete fuses with the polar nuclei, leading to the fusion of three haploid nuclei. This fusion event is known as triple fusion. It results in the formation of a triploid primary endosperm nucleus (PEN).

The central cell, after triple fusion, transforms into the primary endosperm cell (PEC) and develops into the endosperm. The endosperm is a nutrient-rich tissue that nourishes the developing embryo. Meanwhile, the zygote undergoes further development and differentiates into an embryo.

This process, involving both syngamy and triple fusion within the embryo sac, is referred to as double fertilization. It is a unique reproductive characteristic of flowering plants (angiosperms).