Endosperm is:

The correct answer is Option (3) → Triploid

In angiosperms, one male gamete fuses with the two polar nuclei in the central cell. This fusion of three haploid nuclei is called triple fusion. It forms the primary endosperm nucleus, which is triploid (3n). This nucleus divides again and again and forms the endosperm, which stores food for the growing embryo.

(1) Haploid — Incorrect. Haploid (n) cells have one set of chromosomes, such as the egg cell, the synergids and the male gametes.

(2) Diploid — Incorrect. Diploid (2n) cells have two sets of chromosomes. The zygote, formed by the fusion of one male gamete with the egg (syngamy), is diploid.

(3) Triploid — Correct. The endosperm has three sets of chromosomes: two from the polar nuclei and one from the male gamete.

(4) Tetraploid — Incorrect. Tetraploid (4n) means four sets of chromosomes. No normal tissue of the seed has four sets.

NCERT Extract:
Double Fertilisation – “… The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN) (Figure 1.13a). As this involves the fusion of three haploid nuclei it is termed triple fusion. Since two types of fusions, syngamy and triple fusion take place in an embryo sac the phenomenon is termed double fertilisation, an event unique to flowering plants. The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm while the zygote develops into an embryo.”