We know there are no squares between 100² and 101².
So we just want to know how many numbers between a and b exclusive, which is
n = b - a - 1
For example between 10 and 20 exclusive we get n=20-10-1=9, which is right.
For our problem here we can factor using the difference of two squares and get:
n = 101² - 100² - 1 = (101 - 100)(101 + 100) - 1 = 1(201) - 1 = 200
Answer: d. 200