What happens when one of the male gametes fuses with the egg cell? 

Syngamy occurs, forming a diploid zygote

Syngamy occurs, forming a diploid zygote. Option 1 is correct .

After entering one of the synergids, the pollen tube releases two male gametes into the cytoplasm of the synergid. One of the male gametes undergoes fusion with the egg cell's nucleus, completing the process known as syngamy. This fusion results in the formation of a diploid cell called the zygote.

Meanwhile, the other male gamete moves towards the central cell, where the two polar nuclei are located. It fuses with these two polar nuclei, leading to the formation of a triploid primary endosperm nucleus (PEN). This fusion of three haploid nuclei is referred to as triple fusion.

In flowering plants, both syngamy and triple fusion occur within the embryo sac, giving rise to the term "double fertilization," which is unique to flowering plants.

The central cell, after undergoing triple fusion, becomes the primary endosperm cell (PEC), which develops into the endosperm. On the other hand, the zygote develops into an embryo.