From the diagram given, we can see that line a is parallel to line b. This means that the position of each of the angle on line a is equal to the position of each of the angle in line b
From the figure <1 = <(7x+19)degree (corresponding angle). Also <5 = 7x+19 = <1 (corresponding angle as well)
From the above, we can say that <5 = <1 and since <5 and <6 lies on the same stright line, their sum will be 180.
Don't forget that <5 is now replaced as <1. Hence <1+<6 = 180 (supplementary angle).
From the expalnation above, the theorems that was used to prove the relationship between <1 and <6 are supplementary angle, corresponding angle and same side interior angle
Hence option D is correcct