Final answer:
When a circle is divided into 12 congruent arcs, each arc measures 30 degrees, since the total degrees in a circle is 360 and dividing this by 12 gives 30 degrees per arc.
Step-by-step explanation:
If a circle is divided into 12 congruent arcs, you can determine the degree measure of each arc by dividing the total number of degrees in a circle by the number of arcs. Since a circle consists of 360 degrees, you just divide this number by 12, which gives you the measure of each individual arc.
So, the calculation would be 360 degrees / 12 arcs = 30 degrees per arc. Therefore, the correct answer is C. 30 degrees.