What is the maximum area of a triangle that can be inscribed in a circle of radius a?

• A. \(\frac{{3{{\rm{a}}^2}}}{4}\)
• B. \(\frac{{{{\rm{a}}^2}}}{2}\)
• C. \(\frac{{3\sqrt 3 {{\rm{a}}^2}}}{4}\)
• D. \(\frac{{\sqrt 3 {{\rm{a}}^2}}}{4}\)