The total number of nuclei involved during double fertilisation are:

The correct option is C 5
Double fertilisation, an event unique to flowering plants.
It includes syngamy and triple fusion.

During syngamy, out of the two male gametes released by the pollen tube into the embryo sac, one of them fuses with the egg cell resulting in a diploid structure, zygote.

Hence, two nuclei are involved in syngamy.

The other male gamete fuses with the two haploid polar nuclei resulting in the formation of triploid primary endosperm nucleus (PEN). This is called triple fusion.

In triple fusion, three haploid nuclei are involved.

Hence, the total number of nuclei involved in double fertilisation are five.