The correct answer is: (B) Two polar nuclei and one male gamete.
The primary endosperm nucleus is formed during fertilization in plants. It is the result of the fusion of two polar nuclei from the central cell of the ovule and one male gamete (sperm cell) from the pollen. This process is known as double fertilization.
The fusion of the two polar nuclei and the male gamete forms the primary endosperm nucleus, which gives rise to the endosperm. The endosperm serves as a nutritive tissue that provides nourishment to the developing embryo within the seed.
Thus, the primary endosperm nucleus is formed by the fusion of two polar nuclei and one male gamete.