The correct answer is: $180^\circ$
Explanation
In Euclidean (flat) geometry the three interior angles of any triangle always sum to a straight angle, $180^\circ$.
Steps:
- Let the triangle be $\triangle ABC$ with interior angles $\angle A,\angle B,\angle C$.
- Draw a line through $A$ parallel to side $BC$. The angles at $B$ and $C$ become alternate interior angles with two angles at $A$, so those two are equal to $\angle B$ and $\angle C$ respectively.
- Those two angles together with $\angle A$ form a straight line at $A$, so $$\angle A+\angle B+\angle C=180^\circ.$$
Therefore, the interior angles of a triangle always add up to $180^\circ$ (in flat/Eulerian geometry). Note: on curved surfaces (e.g., spherical geometry) the sum can be greater than $180^\circ$.