Triple fusion involves

The correct option is D Fusion of second male gamete with two polar nuclei
The angiosperms are the flowering plants which are known for showing the phenomenon of double fertilization. This is the process in which there are two male gametes. One of the male gamete fuses with the egg cell which results in the formation of zygote. The zygote develops to form the embryo. The other male gamete fuses with two polar nuclei. This fusion results in the formation of the endosperm. Since there are three nuclei, this process is known as triple fusion.

Thus, the correct answer is option D.