A) angle 1 = angle 2 - Incorrect
These angles are only on one of the following lines, which means that they do not show that lines C and D are parallel. In addition, angles 1 and 2 are supplementary, so they could not be equal.
B) angle 1 + angle 2 = 180 - Incorrect
Though this is true, this does not prove that lines C and D are parallel because both of these angles are on the same line.
C) angle 2 = angle 3 - Correct
These angles are alternate interior angles and there are angles on both lines, which proves that lines C and D are parallel.
D) angle 2 + angle 3 = 180 - Incorrect
These angles are equivalent, so they cannot add up to 180 degrees unless the intersecting line was perpendicular to both lines C and D.
Hope this helps!! :)