In a triangle, if one exterior angle is $$120^∘$$​, and the opposite interior angle is $$40^∘$$​, what is the other interior angle adjacent to it?

Correct option is C

Given
Exterior angle of the triangle = $$120^\circ$$​​
One of the opposite interior angles = $$40^\circ$$​​
Theorem Used
Exterior Angle Property of a Triangle: The measure of an exterior angle of a triangle is equal to the sum of the measures of its two remote (opposite) interior angles.
Solution
Let the unknown opposite interior angle be x.
According to the theorem:
​$$\text{Exterior Angle} = \text{First Interior Angle} + \text{Second Interior Angle}\\120^\circ = 40^\circ + x\\x = 120^\circ - 40^\circ\\x = 80^\circ$$

Therefore, the other opposite interior angle is $$80^\circ.$$​​​
Final Answer
So the correct answer is (c)