If A = x% of y and B = y% of x, then which of the following is true?

Answer: Option E

$$\begin{array}{rl}& x\%\text{of}y=\frac{x}{100}\times y\\ & =\frac{y}{100}\times x\\ & =y\%\text{of}x\\ & \therefore A=B\end{array}$$