Haploidy (n) refers to the state of having the half of the set of chromosomes as in a germ cell or a gamete.
Triploidy (3n) is a rare chromosomal abnormality when there is an extra pair of the chromosome in their cells. Example: endosperm in plants.
Therefore, 2n = 20 chromosomes
n = 10 chromosomes
Since endosperm is a triploid (3n). Therefore 3n = 3 * 10 = 30.
So, the correct option is '30'.