"An angle is a right angle if and only if it is not obtuse" can be broken down into "if an angle is a right angle, then it is not obtuse" AND "if an angle is not obtuse, then it is a right angle"
Let's focus on the second statement "if an angle is not obtuse, then it is a right angle"
This is false. One such example is if the angle is 70 degrees as choice B is showing. This is not a 90 degree angle so it's not a right angle, but it's also not obtuse.
