The correct answer is: B. 21st pair.
We can see on the picture that there are three 21st chromosomes instead of normal two. So, it means that the patient has trisomy 21 or Down’s syndrome.
Down syndrome is one of the most common genetic disorders caused by abnormal cell division involving chromosome 21 (failure of the 21st chromosome to separate during the cell division). Symptoms of people with Down syndrome include delayed physical growth, characteristic facial features (Mongoloid face), potential intellectual disability.