To solve this series, let's analyze the patterns in the given equations:
9 = 4: 9 is the square of 3, and 4 is the square of 2.
21 = 9: 21 is the square of 4 + 1, and 9 is the square of 3.
22 = 9: 22 is the square of 4 + 2, and 9 is the square of 3.
24 = 10: 24 is the square of 4 + 4, and 10 is the square of 4.
8 = 5: 8 is the square of 2 + 2, and 5 is the square of 2.
7 = 5: 7 is the square of 2 + 1, and 5 is the square of 2.
99 = 10: 99 is the square of 9 + 0, and 10 is the square of 3.
100 = 7: 100 is the square of 10 + 0, and 7 is the square of 2.
From the given patterns, we can observe that the left side of the equation is always the square of a specific number, while the right side of the equation is the square of a different number.
Applying the same pattern to the equation 16 = ?, we can find that 16 is the square of 4 + 0. Therefore, the square root of 16 is 4.
So, the correct answer is A) 7.