Lin is using the diagram to prove the statement, "If a parallelogram has one right angle, it is a rectangle." Given that $E F G H$ is a parallelogram and angle HEF is right, which reasoning about angles will help her prove that angle $F G H$ is also a right angle? A) Alternate interior angles are congruent when parallel lines are cut by a transversal. - B) Opposite angles in a parallelogram are congruent. C) Vertical angles are congruent. D) The base angles of an isosceles triangle are congruent.

Step 1 ：Given that $E F G H$ is a parallelogram and angle HEF is right.

Step 2 ：In a parallelogram, opposite sides are parallel.

Step 3 ：If we consider the line segment EF as a transversal line, then angle HEF and angle FGH are alternate interior angles.

Step 4 ：According to the properties of parallel lines, alternate interior angles are congruent when parallel lines are cut by a transversal.

Step 5 ：Therefore, if angle HEF is a right angle, then angle FGH should also be a right angle.

Step 6 ：Final Answer: \(\boxed{\text{A) Alternate interior angles are congruent when parallel lines are cut by a transversal.}}\)