whenever a circle is inscribed in a equilateral triangle then we can find out its radius by using formula,

area/semiparameter of triangle

area of equilateral triangle =(3^1/2)/4*Side ^2
=(3^1/2)/4*36
=9*3^1/2

semiparameter of equilateral triangle =3*side/2
=18/2
=9
so radius=(9*3^1/2)/9
=3^1/2
area of circle =3*22/7
hence both are same ,option C