Assertion: Endosperm is a nutritive tissue and it is triploid.
Reason: Endosperm is formed by the fusion of the secondary nucleus to the second male gamete. It is used in developing embryos.
A. Both assertion and reason are correct and the reason is the correct explanation for the assertion.
B. Both assertion and reason are correct but the reason is not the correct explanation for the assertion.
C. Assertion is correct but the reason is incorrect.
D. Both assertion and reason are incorrect.

Hint: The endosperm is a tissue produced inside the seeds of most of the flowering plants which follows the process of fertilization. It is the triploid in most of the species. It also surrounds the embryo and provides nutrition in the form of starch and it also contains oils and proteins.

Complete answer:
The eight nucleate female gametophytes present in the flowering plants have an egg apparatus at the micropylar end which in turn has one egg cell and two flanking cells. The flanking cells are also called the synergids and serve as the site for entry of the pollen tube. The three cells present at the chalazal end of the embryo sac are called the antipodals. One nucleus from each of the poles migrates to the middle and forms the diploid secondary nucleus. The pollen tube penetrates the stylar canal and enters the ovule through the micropyle chalaza or the integument followed by the discharge of the male gametes in the embryo sac. The one male gamete fuses with the egg nucleus to form the diploid embryo. Also, the other male gametes fuse with the diploid secondary nucleus or with the central cell to form the triploid primary endosperm nucleus which after divides to form large nutritive endosperm. Therefore the assertion is correct.
The endosperm serves as the source of food material to the growing embryo and also for the seedling.
Therefore the reason is also correct as the endosperm is triploid because of being the product of the triple fusion between the diploid secondary nucleus and the haploid male gamete.
Therefore, the correct answer is Option (A).

Note: The embryo is surrounded by a nutritive tissue called the endosperm in the seeds of flowering plants. The embryo and the endosperm are derived from the individual fertilization events and develop embedded in the maternal tissues that form the seed coat.