$A$ condition characterized by not having an exact number of chromosomes in a diploid set is called

Sometimes chromosome number increases or decreases due to:

$A$ person who is trisomic for the $21^{st}$ pair of chromosomes has:

What would be the number of chromosomes in the ovum (fertilized by a normal sperm) that resulted in the appearance of Klinefelter's syndrome in the offspring?

What is the cause of $Down's$ syndrome?

In which one of the following combinations $(a - d)$ the number of chromosomes of the present-day hexaploid wheat is correctly represented?