To solve the problem given, let's first understand the statement:

If \(x\%\) of \(a\) is equal to \(y\%\) of \(b\), then mathematically it can be represented as:

\(\frac{x}{100} \times a = \frac{y}{100} \times b\)

By simplifying, we can find the relation between \(a\) and \(b\) as:

\(\Rightarrow x \times a = y \times b\)

\(\Rightarrow a = \frac{y}{x} \times b\)

Now, we need to find what percent of \(a\) is \(z\%\) of \(b\).

Let's express \(z\%\) of \(b\):

\(\frac{z}{100} \times b\)

We now need to find what percent this quantity is of \(a\).

Using the relation between \(a\) and \(b\) from above:

\(\Rightarrow a = \frac{y}{x} \times b\)

Thus, the percentage of \(a\) is given by

\(\frac{\left(\frac{z}{100} \times b\right)}{\left(\frac{y}{x} \times b\right)} \times 100 = \frac{zx}{y}\)

Therefore, the correct answer is \(\frac{zx}{y}\).