The height \( h \) of an equilateral triangle with side length \( a \) can be derived using the Pythagorean theorem. The height splits the equilateral triangle into two 30-60-90 right triangles, where the height is given by:
\[
h = \frac{a}{2} \sqrt{3}.
\]
Thus, the correct answer is \( \boxed{\frac{a}{2} \sqrt{3}} \).