**CONCEPT: **The question tests on the basics of Geometry

**SOLUTION:**

This is a 30-60-90 triangle , its side lengths are always in the ratio of 1: √3: 2. [a: 3√3 a :2a]

Thus, the altitude is 3√3 units.

Area of a triangle = (1/2)*a*b*sin of the included angle

= (1/2) * 3 * 3√3 * sin90

=(9√3)/2 sq units (OPTION D)