If lines \( s \) and \( t \) are parallel, the pair of angles that is supplementary is:
∠A and ∠D because they are consecutive exterior angles.
Consecutive exterior angles formed by a transversal crossing parallel lines are supplementary.
Let's break down the reasoning for the other options:
- ∠G and ∠D because they are a linear pair: This might be true, but it doesn't explicitly involve the concept of parallel lines and their properties.
- ∠B and ∠G because they are consecutive interior angles: Consecutive interior angles are actually not supplementary; they are equal in the case of parallel lines.
- ∠A and ∠B because they are corresponding angles: Corresponding angles are equal, not supplementary.
Therefore, the best choice based on the given options is the one regarding consecutive exterior angles.