Undefined Values of Rational Expression

Consider the rational expression given by:

$$ f(x) = \frac{x^2 - 1}{x^2 - 4} $$

Determine the values of $x$ for which this expression is undefined. The expression is considered undefined when the denominator equals zero. Solve the equation:

$$ x^2 - 4 = 0 $$

and select the correct set of values that make the original expression undefined.