The formula to find the measure of an interior angle is

, where

represents the number of sides in the polygon. In this problem,

would represent

.
Let's plug in the value for


Let's simplify that.

Now we know that the
measure of an interior angle of a regular three-sided polygon is 60°. Your answer would be B!
Let me know if you have any questions regarding this problem!
Thanks!
-TetraFish