Why is fertilization in an angiosperm referred to as double fertilization? Mention the ploidy of the cells involved.

In angiosperms embryo sac, two fusions, syngamy and triple fusion, occur. Therefore, this is known as double fertilization. 

In this process, one of the male gametes moves towards the egg cell and fuses with it, this fusion is called syngamy and it results in the formation of a diploid cell, the zygote, that later develops into the embryo. 

The second male gamete fuses with the secondary nucleus to form a triploid primary endosperm nucleus. This fusion is called triple fusion.

Ploidy of cells : 

Endosperm cell   → triploid

Fertilized egg cell  → diploid

Synergids  → haploid

Antipodals  → haploid