Answer: Choice C) Same-side interior angles
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Angle 4 and angle 6 are on the same side, in this case the right hand side of the transversal line (line t). In addition, they are on the interior of the "train tracks" horizontal lines (line a and line b). Combine this and this is why the two angles are same-side interior angles
Side note: if line a is parallel to line b, then angle 4 and angle 6 add to 180 degrees. At this point, they are considered supplementary.