A right triangle has one angle that's 90° and a corner that looks like an L. Obtuse triangles have one angle that's greater than 90°. In acute triangles, all the angles are less than 90°.
The Pythagorean triple (8, 15, 17) is often seen in algebra and geometry problems. You recognize it as choice A, so you know that is a right triangle.
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A spreadsheet or graphing calculator can perform the tedium of comparing the sum of squares of the shorter sides to the square of the longer side. The attachment shows a spreadsheet used for that purpose. It identifies the triple (8, 15, 17) as the sides of a right triangle.