[tex]\huge\textsf{D.}\ \boxed{79^\circ}[/tex]
[tex]\angle RST[/tex] is an inscribed angle. This means that the measure of the angle is equal to half the measure of the intercepted arc.
[tex]\begin{aligned}m\angle RST&=\frac{1}{2}m\widehat{RT}\\&=\frac{1}{2}(158^\circ)\\&=\boxed{79^\circ}\end{aligned}[/tex]
