Evaluate the following trigonometric function at the quadrantal angle, or state that the expression is undefined. \( \cos \pi \) Select the correct choice below and, if necessary, fill in the answer box to complete your choice. A. The value of \( \cos \pi \) is B. \( \cos \pi \) is undefined.

To evaluate \( \cos \pi \), we can refer to the unit circle. The angle \( \pi \) radians corresponds to 180 degrees, which is located on the negative x-axis. The coordinates of the point on the unit circle at this angle are \((-1, 0)\). The cosine of an angle is defined as the x-coordinate of the corresponding point on the unit circle. Thus, we have: \[ \cos \pi = -1 \] So the correct choice is: A. The value of \( \cos \pi \) is \(-1\).