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)