What is the ploidy of the primary endosperm nucleus?

The correct answer is Option (2) → Triploid

In double fertilisation, one male gamete fuses with the egg to form the diploid zygote. The other male gamete fuses with the two polar nuclei in the central cell. Since this fusion joins three haploid nuclei, it is called triple fusion, and the product, the primary endosperm nucleus (PEN), is triploid.

Option (2) Triploid — Correct. The primary endosperm nucleus is triploid, formed by the fusion of one male gamete with the two polar nuclei.

NCERT Extract:
Double Fertilisation – “… One of the male gametes moves towards the egg cell and fuses with its nucleus thus completing the syngamy. This results in the formation of a diploid cell, the zygote. The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN) (Figure 1.13a). As this involves the fusion of three haploid nuclei it is termed triple fusion.”