To find the angle \( \theta \) in the first quadrant where \( \sin \theta = \frac{\sqrt{3}}{2} \), we can recall the values of sine for some standard angles.
The sine function is equal to \( \frac{\sqrt{3}}{2} \) at \( 60^\circ \).
Thus, the measure of \( \theta \) is:
C. 60°