The circle shown below has ab and bc as its tangents: ab and bc are two tangents to a circle which intersect outside the circle at a point

b. if the measure of arc ac is 160°, what is the measure of angle abc? 80° 40° 60° 20°

Tangent lines of a circle are lines that only pass one point on the circle. Since you forgot to include the picture, I illustrated my own as what the problem described it to be.

One of the theorems of circle is that the measure of the intercepted arc is twice as that of its opposite angle. In this case, since the intercepted arc measures 160°, then the measure of angle abc is 160/2 = 80°. 

Therefore, the answer is 80°.