**CONCEPT: **The question tests the basics of Mensuration and Geometry

**SOLUTION:**

The in-radius of an equilateral triangle is √3 * (side) /6

(In-radius is 1/3 the length of an altitude, because each altitude is also a median of the triangle)

Perimeter of the given triangle =18 units

=>Each side of the triangle =18/3 = 6units [Equilateral triangle]

In radius = √3 * (6) / 6 =√3

Hence area = π r^2

=π *√3 * √3

=3π (OPTION D)