63
81
36
100
Answer is Wrong!
Answer is Right!
The answer is $\boxed{81}$.
The ratio $9:25$ is equivalent to the ratio $\sqrt{9}:\sqrt{25}$, which is equal to $3:5$. Therefore, the ratio $49:x$ is equivalent to the ratio $\sqrt{49}:\sqrt{x}$, which is equal to $7:y$. Since $y$ is the missing number, we can solve the equation $7:y=3:5$ to find that $y=81$.
Option A, $63$, is not the correct answer because it is not a multiple of $7$. Option B, $81$, is the correct answer because it is a multiple of $7$ and it is the only option that satisfies the equation $7:y=3:5$. Option C, $36$, is not the correct answer because it is not a multiple of $7$. Option D, $100$, is not the correct answer because it is not a multiple of $7$.