Final answer:
In mathematics, more specifically in geometry, an exterior angle of a triangle and its adjacent interior angle are supplementary. They add up to form a straight angle, which is 180 degrees.
Step-by-step explanation:
In the subject of geometry in mathematics, when we refer to an exterior angle of a triangle, we are discussing about the angle formed by any side of the triangle and the extension of its adjacent side. The exterior angle of a triangle and its adjacent interior angle are supplementary. This means that both angles add up to form a straight angle of 180 degrees.
For instance, if the interior angle is 60 degrees, the exterior angle would be 120 degrees, because 60 + 120 = 180. Similarly, if the interior angle is 30 degrees, the exterior angle would be 150 degrees, because 30 + 150 = 180. Hence option 'd. Supplementary' is the correct answer.
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