Triple fusion takes place when:

The correct answer is Option (4) → One male gamete fuses with the polar nuclei

Option (1) One male gamete fuses with the one female gamete — Incorrect. One male gamete fusing with the egg is syngamy.

Option (2) Two male gametes with the egg nucleus — Incorrect. Only one male gamete fuses with the egg cell; this is syngamy and gives the diploid zygote. The second male gamete goes to the two polar nuclei, so two male gametes never fuse with the egg.

Option (3) One female gamete fuses with the polar nuclei — Incorrect. Triple fusion needs a male gamete; female gametes do not fuse with polar nuclei.

Option (4) One male gamete fuses with the polar nuclei — Correct. One male gamete fuses with the two polar nuclei, giving the triploid primary endosperm nucleus.

NCERT Extract:
Double Fertilisation – “After entering one of the synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid. 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.”