Chapter 10 - Area - 10-6 Circles and Arcs - Practice and Problem-Solving Exercises - Page 654: 21

90$^{\circ}$

Since the measure of an arc is equal to its corresponding central angle. So, the measure of the arc TB would be equal to the measure of the angle $\angle TOB$, which is shown to be a right angle, equaling 90$^{\circ}$. So, arc TB is also 90$^{\circ}$