Hint:Triple fusion is defined as fusion taking place between two polar nuclei and sperm nucleus.Triple fusion takes place in the central cell of embryo.It results in the formation of endosperm.
This is a process involved in the sexual reproduction of angiosperms.The flowering plants are known for double fertilization.As a result of this there are two male gametes.One of the male gamete fuses with the egg cell which in turn forms a zygote.The zygote develops into an embryo.The second male gamete fuses with the two polar nuclei which results in the formation of endosperm.Finally the above leads to the formation of three nuclei and so this process is called Triple fusion.
Thus the correct answer is option(d)Fusion of the second gamete with two polar nuclei.
The plant kingdom is classified as unisexual flowering plant and bisexual flowering plants.Former catogery forms flowers with either male or female that is they have either stamen or carpel whereas in the later bothstamen and carpel are present.Androceium,the male reproductive part of a flower has an anther and filament.Gynoecium,the female reproductive part of the flower which is either monocarpellary or dicarpellary.It has stigma,styleand an ovary.Pollination is a process in which the male anther transfers pollen grains to stigma.This is carried out by wind or air or by an animal or insect.Pollen tube germination is started after this which penetrates the stigma and grows towards style and reach the base of an ovule.Here double fertilization occurs and thus resulting in the process of Triple fusion.
Note:The main difference between double fertilization and triple fusion is that double fertilization is the fusion of the embryo sac with two sperm cells whereas triple fusion is the fusion of sperm nucleus with the two polar nuclei in the central cell of the embryo sac.