The following picture is a example of a "right triangle" or option B. Right triangles have 90 degree angles and look like a L, if you rotate the triangle to its base you would see that the 90 degree angle looks like a L making it a right triangle. It's not option A because acute angles have to be smaller then 90 degrees in order for it to be acute, it's not obtuse because the angle has to be greater then 90 degrees in order to be a obtuse triangle.
