Right Triangles: Right triangles are known as "right" triangles due to their possession of a right angle, which is in the shape of an "L." Right triangles generally have two sides that are perpendicular.
To answer your question, the answer would be "Two sides that are perpendicular."
As you would know, perpendicular lines generally resemble a plus sign, which is a vivid image of 4 right angles. If you were to add a hypotenuse (the slanted line) to one of the sides, it would make a right triangle, thus answering answer choice B.
Hope you find this answer helpful.