Let's see what the answer choices mean with respect to the requirement to prove a triangle is a right triangle.
a) two sides are congruent: the triangle is isosceles, but not necessarily a right triangle.
b) two sides are perpendicular: the triangle must be a right triangle.
c) three sides are congruent: the triangle is equilateral so is definitely not a right triangle.
d) sides are parallel: the figure cannot be a triangle.
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The appropriate choice is the second one ...
... Use the slope formula to see if any sides are perpendicular.