Question

which triangle is a right triangle?

Explanation:

Step1: Recall right - triangle property

A right - triangle has one angle equal to 90°.

Step2: Check each triangle's angles

In the first triangle, angles are 60°, 60°, 60° (sum = 180°), no 90° angle.
In the second triangle, angles are 45°, 55°, 80° (sum = 180°), no 90° angle.
In the third triangle, angles are 35°, 35°, 110° (sum = 180°), no 90° angle.
In the fourth triangle, angles are 30°, 60°, 90° (sum = 180°), has a 90° angle.

Answer:

The triangle with angles 30°, 60°, 90° is a right - triangle.