The correct equation that shows the conservation of mechanical energy for a marble attached to a compressed horizontal spring when released is:
\(\frac{1}{2}mv^2 = \frac{1}{2}kx^2\)
This equation represents the conversion of potential energy stored in the spring (when it is compressed) into kinetic energy of the marble as it is released and starts to move.
Thus, the correct response is:
\(\frac{1}{2}mv^2 = \frac{1}{2}kx^2\).