To evaluate the expression \((21x^{-2})^0\), we can apply the zero exponent rule, which states that any non-zero number raised to the power of zero is equal to 1.
Therefore, \((21x^{-2})^0 = 1\).
Now let's analyze the given responses:
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\((21 x^{12} y)^1\) - This is equal to \(21 x^{12} y\) (not equivalent to 1).
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\(0 \times x^2 \times 21 x^{4}\) - This equals 0 (not equivalent to 1).
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\(3x^{2} \times \left(\frac{1}{7}\right)x^{-2}\) - This simplifies to \(3x^{2} \times \frac{1}{7} \times \frac{1}{x^{2}} = \frac{3}{7}\) (not equivalent to 1).
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\(\frac{7x^{2}}{7x^{2}}\) - This simplifies to \(1\).
Thus, the expression that is equivalent to \((21x^{-2})^0\) is:
\(\frac{7x^{2}}{7x^{2}}\).