To determine which logarithmic expression is undefined, we need to remember that the logarithm of a non-positive number is undefined. The logarithmic expression \(\log_b(a)\) is defined for \(a > 0\) and \(b > 0\) (with \(b \neq 1\)).
Now let's evaluate each of the given expressions:
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\(\log_3(-9)\): This is undefined because the argument \(-9\) is negative.
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\(\log_{0.25}(64)\): This is defined because \(64 > 0\), and \(0.25 > 0\) with \(0.25 \neq 1\).
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\(\log_{2.5}(6.25)\): This is defined because \(6.25 > 0\), and \(2.5 > 0\) with \(2.5 \neq 1\).
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\(\log_3(27)\): This is defined because \(27 > 0\), and \(3 > 0\) with \(3 \neq 1\).
So, the expression that is undefined is:
\(\log_3(-9)\).