Final answer:
The phrase "In a plane," is necessary to ensure that the context is two-dimensional Euclidean geometry, where the Corresponding Angles Theorem indicates that a transversal perpendicular to one parallel line must also be perpendicular to another.
Step-by-step explanation:
The correct answer to whether the phrase "In a plane," needs to be included for a statement to be true is C. The phrase needs to be included because it restricts the context to two dimensions, which is essential when talking about parallel lines and transversals in Euclidean geometry. By stating "In a plane," it is ensured that the two parallel lines define that plane, and any transversal that is perpendicular to one of these lines will also be perpendicular to the other. This follows from the Corresponding Angles Theorem, which implies that alternate interior angles are equal when a transversal intersects two parallel lines in a plane. When one of these angles is a right angle, so is the corresponding angle, confirming that the transversal is perpendicular to both lines.