Since the triangle will be a 30-60-90, because 360°/6 = 60° for each central angle, then that is bisected to find the height of each of the 6 equilateral triangles. This height is the same as the apothem.
In a 30-60-90 triangle, the sides are is the ratio of:

where x is opposite the 30° and 2x is opposite the 90°
Since the base is 10 and the hypotenuse is 10, 1/2 that is 5 (the x), and so:

So rounded up, you apothem then is closest to D) 9 in.