When the root tip cell has $2n=20$ chromosomes, then the number of chromosomes in the endosperm will be
A. 10
B. 20
C. 30
D. 40

Hint: The endosperm is the outer protective covering of the seed that provides its nutrition during the germination period. It has an extra pair of chromosomes in it. It shows a rare chromosomal abnormality. The endosperms are in a triploid state.

Complete step by step answer: Ploidy refers to the number of sets of chromosomes present in a cell nucleus. A haploid cell means it will have only one set of chromosomes or $n$ chromosomes. A diploid cell or $2n$ cell is that which will have two sets of chromosomes, one from the maternal gamete and the other from the paternal gamete. The root tip cells from the root cap in roots. They are diploid, meaning $2n$. They have two sets of chromosomes, one from the ovum and one from sperm. The endosperm is a tissue that surrounds the seed and provides nutrition to it. It shows a rare chromosomal abnormality that makes it triploid in many species. The endosperm has an extra set of chromosomes along with $2n$ sets of chromosomes.
Now, as it is clear that the endosperm will have three sets of chromosomes, thus it will have $n\times 3$ number of chromosomes. The number of chromosomes in the endosperm with triploid state or $3n$ state is $10\times 3=30$ that is $3n=30$. Thus, this is the correct answer.
Hence, the right answer is option C that states that if root tip cells have $2n=20$ chromosomes, then the number of chromosomes in endosperm will be $10\times 3=30$.

Note: The term ploidy and what significance it has in calculating the number of chromosomes. Triploid is a rare state as compared to haploid and diploid. Endosperm exhibits a triploid state and provides nutrition to the seeds in the form of starch. It also consists of oils and proteins. Wheat and barley endosperm are used by humans for their nutrition like in bread and beer respectively.