Hi Folks. Can someone explain why the answer for this A and not D. Based on the properties of an inscribed triangle in a circle, I know that the measure of angle RTS is 90, but why does the angle RXS have to be acute (i.e. < 90) and cannot be right or obtuse?

You can imagine a point which is very far from the center. The measure of this angle will be negligible.

We can see that the angle subtended by the diameter is max at the center which is 180. This decreases to 90 at any point on the circle and then continues to decrease in like fashion.

Therefore, any point farther than the radius will have a measure of less than 90.