In a regular pentagon, the relationship between the radius (R) of the circumscribed circle and the side length (s) can be calculated using the formula:
\[ s = R \times 2 \times \sin\left(\frac{\pi}{n}\right) \]
where \(n\) is the number of sides (for a pentagon, \(n = 5\)).
Here, the radius \(R = 12\) mm. Plugging in the values:
\[ s = 12 \times 2 \times \sin\left(\frac{\pi}{5}\right) \]
Calculating \(\sin\left(\frac{\pi}{5}\right)\):
\[ \sin\left(\frac{\pi}{5}\right) \approx 0.5878 \]
Now, substituting back:
\[ s = 12 \times 2 \times 0.5878 \approx 12 \times 1.1756 \approx 14.1 \text{ mm} \]
So, the measure of each side of the pentagon is approximately 14.1 mm.
The closest provided option is 14.1 mm.