In angiosperms , which of the following structure always precedes the development of embryo?

Formation of endosperm.

The correct answer is option (4) -Formation of endosperm.

During double fertilisation in angiosperms, one male gamete fuses with the egg to form the diploid zygote (syngamy or generative fertilisation). The diploid zygote finally develops into embryo. The other male gamete fuses with the two polar nuclei(or secondary nucleus) to form the triploid primary endosperm nucleus, PEN (triple fusion or vegetative fertilisation). double fertilisation ensures that the nutritive issue is formed before the zygote starts cleaving so that the energy spent on the formation of endosperm does not go waste in case the fertilisation fails. Angiosperms are therefore, economical and more specialised as compared to gymnosperms where a large nutritive female gametophyte is formed long before fertilisation.

The other structures mentioned in the options may or may not be present in all angiosperms and do not always precede the development of the embryo:

Formation of nucellus: The nucellus is a part of the ovule that contains the embryo sac. It does not always precede the development of the embryo, as the nucellus may or may not be present in all angiosperms.

Formation of perisperm: Perisperm is a nutritive tissue that surrounds the embryo in some plant species. It can develop from the nucellus or from the fusion of the nucellus and the integument, but it is not always present, and its formation is not always a prerequisite for embryo development.

Formation of seed: The formation of the seed occurs after the development of the embryo and is the result of the maturation of the ovule. It involves the transformation of the ovule into a seed, which includes the development of the seed coat and the storage tissues.

Therefore, among the given options, the formation of the endosperm is the structure that always precedes the development of the embryo in angiosperms.