what is the measure of angle coa?\n140°\n150°\n160°\n170°

Explanation:

Step1: Recall the central - inscribed angle relationship

The measure of an inscribed angle is half the measure of the central angle that subtends the same arc. Angle $\angle B$ is an inscribed angle and $\angle COA$ is the central angle subtending the same arc $\overset{\frown}{CA}$.

Step2: Apply the formula

If $\angle B = 80^{\circ}$, and $\angle B=\frac{1}{2}\angle COA$ (by the inscribed - central angle theorem), then $\angle COA = 2\angle B$. So, $\angle COA=2\times80^{\circ}=160^{\circ}$.

Answer:

C. $160^{\circ}$