The tangent function is undefined when the cosine of the angle is zero, since the tangent function is defined as the ratio of sine to cosine (\( \tan(\theta) = \frac{\sin(\theta)}{\cos(\theta)} \)). Therefore, we need to find angles where \(\cos(\theta) = 0\).
The angles where cosine is zero in degrees are:
- \(90^\circ\)
- \(270^\circ\)
From the given options, \(90^\circ\) and \(270^\circ\) are the angles at which the tangent function is undefined.
Therefore, the correct set of values is at 90 and 270 degrees.